I'm actually expecting a minor flood of apps to start pouring in (that's not technically a mixed metaphor, is it?).
First, there are many more developers who've been admitted to the early access program than have released applications.
Second, we had a large number of updates available immediately following 1.0.4 that I believe were just waiting for the update. I think there's another update coming soon to address some specific problems such as any remaining EAS sync issues (but not the missing features), performance issues, Exchange-Outlook reply formatting issues (and, I think, formatting issues in general since my Exchange email won't show as HTML), etc. Once that update is released, I think they'll release another round of applications that have been waiting in the wings.
Or, Second (alternative version), they're revamping some aspects of the App Catalog that will be released relatively soon, and once that's done then a second round of apps will be released.
Whichever #2 is correct, a dedicated Facebook app will be part of the next round. It's hard to believe that we don't have one yet, and I can't imagine that it'll be much longer.
Note that this is all idle speculation. And, I'm still pretty sure that the Pre will be launched on May 17...