I'm out on a limb here, but here's my opinion: The creature with earth glide should hit the ground and be dealt normal falling damage. That's because...
A creature with the special ability earth glide should have a burrow speed because earth glide is just fancy burrowing
...and having a burrow speed—even when combined with earth glide—doesn't protect from falling damage. The DM may rule, given earth glide's flavor text, that diving into the earth is the equivalent of the creature falling into water, but that's still house rules territory.
In fact, I suggest any creature without a burrow speed yet possessing the special ability earth glide (like the earth elemental) should be house ruled to have burrow speed equal to its speed (like the xorn) because otherwise earth glide's functionality is almost entirely at the DM's whim.
This eliminates the guesswork involved with the special ability earth glide—Is it like swimming? Is the speed the creature's land speed? Can the creature breathe?—, making earth glide instead function just like burrowing except that it looks cooler and goes through more stuff easier. Under this house rule, while it would be awesome for a creature with the special ability earth glide to be pushed off a cliff and dive into the earth instead of smashing into it, such a creature can no more do that than could a common real-world mole.1
I recommend this because the extraordinary ability earthglide says
A [creature] can glide through stone, dirt, or almost any other sort of earth except metal as easily as a fish swims through water. Its burrowing leaves behind no tunnel or hole, nor does it create any ripple or other signs of its presence. A move earth spell cast on an area containing a burrowing [creature] flings [it] back 30 feet, stunning the creature for 1 round unless it succeeds on a DC 15 Fortitude save.
Boldfaced emphasis mine. "But," you ask, "doesn't that description just make earth glide a form of burrowing?" Ah, but were it, there'd be no reason for the xorn to have both earth glide and burrowing. (The xorn's burrowing is the standard kind and far less versatile than the special ability earth glide, its burrowing being unable, for example, to penetrate rock. Even redundancy isn't an issue; the xorn's earth glide is an extraordinary ability anyway.)
If that's convincing enough for you, and you don't care how I got from there to house ruling in an entire movement mode to an iconic creature, that's cool. You can stop reading. Otherwise, though, we must have a serious conversation. About xorn. In fact...
Let's go back to the turn of the century...
In Dungeons and Dragons, Third Edition the earth elemental didn't possess the special ability earth glide. Instead, like there is now, under the earth elemental's Combat heading but before the description of the earth elemental's special abilities—which, in 2000, we're only the extraordinary abilities earth mastery and push—there's this:
Though an earth elemental moves slowly, it is a relentless opponent. It can travel through solid ground or stone as easily as humans walk on the earth's surface. It cannot swim, however, and must either walk around a body of water or go through the ground under it. An earth elemental can move along the bottom of a body of water but prefers not to. (Monster Manual (2000) 83 and Monster Manual (2012) 98)
But that was it. The earth elemental didn't burrow or anything, and there was no ability type for this. In fact, you could totally miss this if you were throwing an earth elemental at the party as, like, a random encounter or a summoned monster or whatever. Unless you knew to look there—in a spot usually reserved for flavor text or tactics or whatever—the earth elemental was just a big pile of hard-hitting hit points instead of an interesting tactical encounter. That ability simply isn't reflected anywhere else in the earth elemental's creature entry.
...then to 2003...
With Dungeons and Dragons 3.5 revision, the Monster Manual underwent a pretty serious overhaul, and some that book's stealth rules—the sort of seat-of-your-pants, make-it-up-you're-the-DM! rules that were common in earlier editions and even Third Edition's own early texts2—were converted into actual rules, and the earth elemental's travel-through-solid-ground text appears to be one such victim. It seems Richard Baker or Andy Collins or somebody who worked on the Monster Manual revision thought the unnamed-and-untyped earth elemental walk-through-earth ability sounded a lot like the xorn's extraordinary ability burrow:
A xorn can glide through stone, dirt, or almost any sort of earth except metal as easily as a fish swims through water. Its burrowing leaves behind no tunnel or hole, nor does it creature any ripple or other signs of its presence. A move earth spell cast on a burrowing xorn flings the xorn back 30 feet, stunning the creature for 1 round unless it succeeds on a Fortitude save. (Monster Manual (2000) 188)
So the reviser pasted that description into the earth elemental description, changed a few words, renamed it earth glide, and clocked out... without realizing an earth elemental has no burrow speed! It's an easy assumption to make: I mean, of course an earth elemental can burrow! Besides, it's quittin' time, and in a couple of years this whole d20 thing'll blow over, and nobody'll care.
...then to Underdark, Races of Stone, and beyond
Post-Monster Manual 3.5, other authors, noting the earth elemental's earth glide yet lack of a burrow speed and ignoring the less popular xorn with both, also gave their creatures the extraordinary ability earth glide without giving their creatures a burrow speed. For example, Underdark gets it (in my opinion, anyway, sort of) right with the appropriately named earth glider (84) (I kid you not—"Speed: 20 ft. (6 squares), burrow 50 ft.") but also has the stone flyer (99) with the special ability earth glide and earth glide rider but no burrow speed.3
This gets crazy by Races of Stone (Aug. 2004) with the hammer archon (188-9) having earth glide but no burrow speed, the 8th-level Sor/Wiz spell earth glide (162) allowing a creature to earth glide but not granting a burrow speed, and two prestige classes granting the supernatural ability earth glide: the stone speaker guardian (127-8) has earth glide that specifies that the creature moves while using it at half its speed, but the earth dreamer (110-2) has no such speed indicator.
The last new mention of earth glide that I could find comes in Complete Champion (May 2007) as part of the paladin alternative class feature underdark knight, which says that in exchange for the spell-like ability special mount
At 12th level, you gain the earth glide extraordinary ability, which enables you to move through stone, dirt, or almost any other sort of earth except metal as easily as a fish moves through water (MM 98). (49)
That Monster Manual page reference? It points to the earth elemental's extraordinary ability earth glide, leaving the xorn—whose ability it was in the first place back in 2000 and who is now apparently accidentally stuck with a useless burrow speed—forlorn.
Earth glide post-mortem: Pathfinder noticed
Someone over at Paizo presumably noted this disconnect, and the D&D 3.5-derived Pathfinder Role-playing Game revised the special ability earth glide, tying the special ability directly to a creature's a burrow speed, and that game's xorn and its earth elementals have burrow speeds.
1 To be that cool, such a creature takes the ready action and picks the condition When I hit the ground and the action I use earth glide to burrow. The condition met, the creature will disappear into the earth. A generous DM may even allow the creature to fall (i.e. drop prone) off the cliff as a free action instead of waiting to be bull rushed.
2 Although it may sounds like it, that's not in any way disparaging. Monsters of Faerûn (2001), for example, is a hoot.
3 How fast can a stone flyer earth glide? Fortunately, both its speed and fly speed are 50 ft., so we can guess that it's probably supposed to be 50 ft., but, without a burrow speed, who knows?