It's fairly easy to make fast friends through social networking, because there are plenty of others looking to do the same. However, I think the real goal should always be to build quality relationships. That doesn't mean you have to be best friends with each and every one of your followers/friends, but you should at least have some reason for being their friend other than padding your numbers. Those people are more likely to be your evangelists and tell even more of their friends about you, widening your circle even more.
When I first got started in social media, I went for the fast and furious approach to networking, but now I've slowed my roll a bit. A news feed clogged with stories and people you don't have anything in common with is a drag, no matter how high your follower or friend count.
I think that the best way to make and keep friends is just to produce high quality content, *often*. This goes for a well-written FB status the same as it does for a professional blog post. Be confident, reach out to others and don't just wait for them to come to you. If you see a profile you can relate to for one reason or another, strike up a conversation based on something yo both have in common.
Also, ask questions about the other person. This works in social media as well as it works in real life. Everyone's favorite subject is them, after all.