In grad school I had a friend I had a mild crush on. She posted on Facebook that she was going to a concert by this over-the-top disco band -- "Anyone want to come along?"<p>I did. Not only did I like this person, but ridiculous disco music is very much my idea of a good time! We had a blast.<p><i>Maybe</i> that evening happens without FB, maybe not. FB offers a way to ping your whole social network and see what comes back. Sometimes nothing -- which can be a downer. But sometimes it works, and when it does it's awesome.
It's a great way to keep track of the latest news and happenings in the world and in my local community. I admit I wasn't much of a news reader when I was younger, but now I'm much more plugged in and social media helps me do this.
I have used it to contact people who you haven't contacted in a while without their current contact information on hand. In other words, it provides identity services that may be less brittle than e-mail or phone numbers.
Momentum. If an issue is identified and numerous people seem to care about it and pass it along, few channels can match a social network's ability to get it attention.