Ben Parr writes a column for Mashable called The Social Analyst. Here is an exceprt from his comparison of Facebook and Twitter:

“On Facebook, you’re supposed to connect with close friends. Becoming friends with someone means he or she gets to see your content, but you also get to see his or her content in return. On Twitter, that’s not the case: you choose what information you want to receive, and you have no obligation to follow anybody. Facebook emphasizes profiles and people, while Twitter emphasizes the actual content (in its case, tweets).”

“The result is that the stream of information is simply different on both services. You’re more likely to talk about personal issues, happy birthday wishes, gossip about a changed Facebook relationship status, and postings about parties on your Facebook News Feed. On Twitter, you’re more likely to find links and news, and you’re more likely to follow brands, news sources and other entities outside of your social graph. In fact, Twitter tells me that one out of every four tweets includes a link to some form of content.”

I think if you boil it down, for me it’s the difference between “Friending” and “Following”

“Unlike most social networks, following on Twitter is not mutual. Someone who thinks you’re interesting can follow you, and you don’t have to approve, or follow back.”