We've had a forum for our group for years that players could use to post both in and out of character. It's nothing as immersive as a full wiki like Obsidian, but I think a forum setting well represents the base level of getting players involved in online building up of your campaign.
What we've found is that some players take a very active roll in participating in the forum, posting frequently and some of us have very in depth in character conversations in between game sessions (we game weekly). Other players almost never participate in the forum. I think you'll probably find that this behavior will be common across most groups.
We take turns being DM/GM and each of us has approached this a little differently. Some have offered minor XP or other bonuses of various types to try and encourage participation in the forum. But in general I think we've found that offering bonuses really doesn't change who takes an active participation and who does not. I'm sure your mileage will vary, but I would bet that you'll find most groups will turn out like this. Some will participate without extra incentives, some won't participate much no matter what, and you may get one in the middle who can be swayed with little bonuses.
Obsidian Portal, however, with it's numerous features ready made for game groups, may provide more motivation for groups to participate more fully since they can expand upon much more than just character interaction in a forum. And it's an advantage for the DM/GM, allowing them to add their own notes to the various pages that only they can see. The downside of that is that for groups like the one I'm in where the GM/DM rotates, it doesn't really work well. A group would have to set up multiple portals in order to have different sections that only the DM of that section has control over.
At least, this was the case last time I looked into Obsidian. That was a year or more ago and may have changed since. For a group like mine, where we have rotating DMs but essentially are all operating in the same campaign world, it's not quite right for our use. It may be great for yours.