From IFWiki

Facebook (http://www.facebook.com) is a social networking service where people can set up a profile page, link up with other Facebook members as friends, join groups where people have common interests, send each other free gifts, write messages on each others' walls, post notes, photos, and videos, play games, and so on.

Each Facebook group has its own forum where members can post to various discussion threads. Here are some of the known Facebook groups related to interactive fiction:

And there are also some "fan" pages, which are similar to groups, but perhaps a bit more suited to help businesses notify their fans about their product, and find out what their fan base is like: