Some people scoff at those who have too many friends on Facebook. "They probably just friend a bunch of people they don't know to look popular," they say. While I'm not interested in having lots of 'friends' for that reason, I do find that Facebook can be incredibly valuable for networking. That's why I tend to have 400-500+ connections at any given time.
You know how when you used to go to a conference and you met someone you were interested in connecting with? The standard practice would have been to exchange business cards. I have no idea how often people would have corresponded before email, but before social media, email was the main choice. I remember emailing people after some conferences in my early undergrad years. We'd exchange one -- maximum two -- emails and then forget about each other.
One day in early 2007, someone I met at a conference finally convinced me to join Facebook. I had been avoiding it because the concept seemed dumb at the time (shows how much I knew), but finally relented so I could keep in touch with this person and a few other conference attendees.
This kind of networking is still probably one of my most valuable reasons to have Facebook to this day. Now if I want to keep in touch with someone, I find them on Facebook instead of thinking I'll actually email them more than the first time. I can have a passive connection with them where neither of us have to put any extra work into keeping in touch, but we don't forget about each other. Plus, if I see an update from one of these people that I think I can help out with, I jump on the opportunity. Most do the same for me. I've definitely seen many of them again thanks to this!
(Note: This goes for Twitter or any other social network that you and the other person you are connecting with use often. Take advantage of the places you hang out anyway!)