What do you think is Facebook's long-term strategy?
"We want to make Facebook into something of an operating system so you can run full applications..." (Zuckerberg). Ok that's great, but the majority of Facebook apps are FAR from what a traditional OS offers.
As a successful app developer (550k users) myself, the bin of roughly 9,000+ growing apps has made it pretty damn hard to unite any major Facebook usergroup behind a single app. It's the same problem as trying to find a single, unified, interest-group among the millions that exist (hint: you can't). If only 2 of your 200 friends have added the app, it's pretty much worthless from a SocialOS standpoint, no matter how awesome it is. Obviously "social apps"-"social"="plain old app", and there are very few Web apps that can compete with plain old desktop apps right now.
Facebook has executed the social network component amazingly well, but their vision as a SocialOS by web developers has not been much more than a hack-job with 99% of apps falling into the "Value-Added: None" category. They would do better to buy up the good applications by developers and make them permanent components to Facebook.
Don't even get me started on the SocialAds network either, that has absolutely ZERO benefit to the end-user other than another way to brag about what you've bought to your friends.
Right now the biggest thing Facebook has for it is the appeal of FB as the cool new thing to do (over Myspace) and the fact that many of us still use it as a glorified contacts directory (College student speaking here who's been on FB since v1).
So in summary, we have a great Social Network + Poorly executed socialOS + Privacy-Invading Ad Network. Right now users are only benefitting from the first one, and I see no reason why they won't just move on to the next great(er) social network. After all, that's exactly what's happening as people get rid of Myspace for Facebook.