Facebook is finally giving up on the primacy of friendship with the launch of the "Subscribe" button. Now, instead of being stuck with the option of adding someone as a friend, you can subscribe to their updates. (Just what Facebook users need: another button to express ambiguous feelings and desires.) The feature is opt-in, more complicated than it sounds, and yes, a lot like Twitter.

Adding a subscribe button to your profile is dead simple. Just go to this page and click the green button. You'll then be prompted to choose who will be able to comment on your posts to subscribers; "Friends of Friends" is the default. Now whenever you post something publicly on Facebook, the update will be sent to your subscribers as well as your friends. A "Subscribers" link will pop up on the lefthand side of your profile, where you can then tweak various setting like who's allowed to subscribe to your updates, who's allowed to comment and what prompts notifications. Since you already see their updates, you can't subscribe to your friends, and people that request to add you as a friend also now automatically subscribed to your feed.

The Subscribe button is really geared towards the subscribers though. Based on the wording in their announcement blog post, Facebook recognized that one-way friendships were limiting and hinted at wooing Twitter's most sacred users: celebrities. "Until now, it hasn't been easy to choose exactly what you see in your News Feed," writes Zach Rait. "You also couldn't hear directly from people you're interested in but don't know personally--like journalists, artists and political figures." Unlike Twitter, however, Facebook lets you customize the crap out of what you see from to whom you subscribe as well as your friends. Along with the launch of the Subscribe button, Facebook has unveiled a bunch of new options that let you fine-tune how items show up in the feed. Do you only want to see "life events" from your high school friends from Tennessee? How about turning off all of the annoying FarmVille updates from your cousin with too much time on her hands? Do you prefer a photo-only feed? The Subscribe button makes it all possible.

We're tempted to go on a longer rant about what the Subscribe button means for Facebook's future. As our own Rebecca Greenfield pointed out recently, it's hard to tell what kind of company Facebook wants to be. But it's defintely safe to say that Facebook is creeping deeper into Twitter's territory. MG Siegler found some hints deep in the documentation for developers on the Subscribe button that you'll soon be able to tweet directly from your Facebook profile. So what can't you do on Facebook now--besides make fun of Republicans?