"Facebook is one of the largest social networking sites on the Internet. It currently has roughly 600 million users around the world. Users create a profile page and then connect it with the profile pages of their friends and family. They can share status updates, event invites, photos, videos, notes, and links to other websites."--from LDS Media Library
I have an account on Facebook so I noticed this article published by the LDS Media library. You can find it by clicking on the photo above. It gives a good overview of Facebook. It particularly tells how the LDS church uses the social media site. But I want to stress that the site is a valuable resource for genealogy too.
I've found nearly 100 relatives on Facebook, all of whom I had no contact with prior to finding them there. I've been able to gather family photos, significant dates (births, anniversaries, etc) from their information, and even have conversations with members of my family with whom I'd lost contact over the years, or just never even knew. Just this past week my second cousin, whom I haven't met in person, posted a picture on Facebook of our common great grandfather. I'd never seen the photo, but was able, with her permission, to save it to my computer and add it to my collection! What a wonderful opportunity!
Since I author several blogs, I've also connected with several hundred other "Genea-Bloggers" on Facebook. We share links to resources, news from the genealogy field, and offer support and encouragement to each other in the group. I learn of conferences, podcasts, webinars, and personal appearances of experts in the genealogy field.
I encourage you to read the article that I've referenced above at LDS Media Library and check out Facebook if you don't already use it.