Facebook is the premier online social network, with over 200 million users. Since 2007, developers have been using the Facebook Platform to design applications on Facebook that can work with Facebook data. Then, with the introduction of Fan Pages, Facebook became available to businesses as a rich and valuable marketing medium, made more powerful with custom Facebook applications. Cool applications for Facebook can bring in millions of dollar a year when monetized correctly; or, you can buy Facebook applications that help engage an audience, reach new users, and promote your brand for long-term benefits.
Its making the most out of your signature; One of the best forum-advertising methods Back ontopic: The use of facebook apps to promote your site depends on the niche, quite useless to have if your app is based on something else compared to your site.
100% correct but on the other hand, since they allow you to monetize it with your own ads you can still do very well with them.. But it is starting to get to that over-saturation point where there are so many clones and crappy apps out there, unless you have a unique idea or really know how to promote your app you are not going to reach the bizillion people on facebook
Facebook promotes almost anything, and with the number of members it become powerful in promoting your sites