On the one hand there are downsides and risks being tied to a platform, but on the other hand there are huge opportunities. For applications that are inherently social or have a strong social component, it's going to be very valuable or necessary to live on the social graph. Until now, those companies have had to try to build their own, which is very difficult or impossible.

For now a lot of developers just see Facebook as a way to spread virally, which has led to a lot of annoying spam machines.