Facebook has become more of an encumbrance than a benefit for me. Here's why I quit:
I never was a power user. Like millions of other folks, I used Facebook on occasion, but it never displaced email and IM as my primary means of online communication. Something about putting all of my eggs in one basket always unnerved me.
I found it too difficult to keep the chocolate and peanut butter separate. Sometimes you want all of your friends to see your messages. Sometimes you want a subset of your friends to see your messages. While there are ways to segment your social network into sub-networks (and sub-sub-networks?), that was too much effort for me. Old school email may be boring, but it is quite efficient for this sort of thing.
I had a lot of friends who weren't friends. I had Facebook friends who really would be better placed in the I'm Glad You're Still Alive And Gosh Wasn't High School Fun But We're Not Really Friends Anymore category. I've had some enjoyable conversations with old friends and acquaintances from back in the day, but unfortunately those interactions were the exception to the high-chaff norm.
I got tired of invitations. Walking down Pacific Avenue every morning I have to run the gantlet of people with their hands out for change, people wanting me to contribute to various splendid causes, and people wanting me to listen to their deranged rants. If I wanted that online, I'd turn off my spam filter.
I got tired of seeing messages that weren't really intended for me. I wasn't the only Facebook user having difficulty with boundaries. Frankly there are some bits of information about friends and acquaintances that I don't need or want to see. Facebook makes it easy for users to broadcast that which should be narrowcast.
The interface made me feel like I was inside a shopping mall. I hate shopping malls. They are designed to lure you in and keep you there. Facebook was like that, but worse. It was a mall comprised of storefronts that magically shifted location between visits. To Facebook aficionados this is probably not a big deal. As a casual user, it was a big deal.
I started to wonder if thin and frequent communication was really preferable to infrequent but substantial communication. I could see lots of little points of activity from my Facebook friends. Over time I began to form impressions of these people that were based solely on this passive interaction. I came to believe that I knew what these people were about. In actuality I was seeing narrow samples of these people. The samples weren't random, either. Some Facebook friends tended to post when they were happy, some when they were sad, some when they were bored, some when they were proud. When I communicated with them via email, phone, or face to face, I found that most of the time the impression I'd formed by way of Facebook was inaccurate. That made me wonder what sort of skewed picture of my own life I was presenting through Facebook.
I came to distrust Facebook, the company. I don't wear a tinfoil hat. I don't use OpenBSD on my laptop, and I doubt I've ever used an ampersand in a password. But I don't like dealing with a company that has repeatedly shown its contempt for customer privacy. The unspoken motto at Facebook seems to be, "Whatever we can get away with." I give companies access to personal information all the time, but only when I believe the gain in doing so outweighs the loss of control over that information.
Email is still the de facto standard for online communication. In ridding myself of Facebook, I am making it slightly more difficult for some people to contact me. But email isn't going away, and the future of Facebook is by no means assured. Just ask any MySpace employee.