Social networks keep gay people from having the same, uncomfortable conversation over and over again. By coming out on Facebook, people no longer have to tell all their friends at different times that, yes, they are in fact gay.
I remember the apprehension I had prior to going to a high school reunion in the days before Facebook was popular. I didn't care who knew I was sapphically inclined. I just resented having to tell them.
Now she assumes that when she becomes friends with people on Facebook, granting them access to her profile, she's effectively come out to them.
Facebook provides love-life announcement services to heterosexual users as well — for better or worse. I've learned about my acquaintances' new relationships, engagements, and breakups after the updates showed up in my news feed.