* They could compete with Groupon (Or buy them). Groupon is an inherently social business, and Facebook can enjoy it. * They could compete with Skype and other VOIP service. Perhaps replacing telephony. Almost everyone has facebook, and you already have you friends inside, seems like a pretty direct step (After all, people come to facebook to communicate). * They could replace photo sharing websites (Flickr, Picasa...) if they just improved their photo's app (e.g. being able to view high resolution images). * They have the potential to become the internet ID for everyone, so that you could log in to almost any site with facebook.
I can probably come up with many more... I think the real question is whether Facebook can execute or not.