There is no facing by default
Facing is an optional rule in D&D 3.5. You can see this from it being a variant rule, that is introduced as follows:
The standard d20 combat rules intentionally ignore the direction a creature faces. The rules assume that creatures are constantly moving and shifting within their spaces, looking in all directions during a fight.
Because there is no facing, there is no effect of being immobilized by web on facing. Immobilized is also not a formal condition.
It would be up to the DM to decide if you can or cannot change orientation if stuck in a web for other purposes.