We knew that Facebook had bigger plans for vanity URLs/usernames when it opened them up for registration this summer.
The popular bet was that it would create a mention/reply system similar to Twitter's @replies. That was shot down this week when Facebook launched its mentions system, which uses real names. But on Thursday, Facebook started letting users log in with their usernames.
This might not seem like a big deal at first glance since most Web sites use username log-in. But this is a time saver, compared with typing in your full e-mail address every time. I still suspect that we will see our Facebook usernames being used for more things in the future, though it's unclear what those things may be.