I stopped using Facebook years ago, it's not that uncommon. At first it was a compulsive sever. As time passed, my lack of interest validated the decision.
I have nothing against Facebook personally. I keep a blank test account whenever I take on work for a Facebook app. I also like to keep abreast on the innovative UI techniques their engineers develop.
Being on a computer for 10+ hours a day, having a mini-computer in my pocket at all times and an iPad for everything inbetween means I'm always being drawn to email, games, articles or something on truly irrelevent.
This lifestyle means I've had to consciously decide how I use technology. I have conventions for; sending email, sharing files or posting photos. It's too easy to always be online.
I Like to Like the Real World
It's a sophomoric argument to make because Facebook is filled with real people in real photos and engaged in real conversation. I just find it more enjoyable to be unworried about checking in my location, or immediately posting a photo to Facebook.
I like experiencing where I am at without worrying about letting everyone else know what I am doing.
I once heard (link:https://twitter.com/mg text:Matt Galligan) answer a question about (link:http://pleaserobme.com/ text:location privacy) at a mobile conference. His answer was simple, people have been listed in the phone book for years and most go to work from 9-5. If someone wanted to burglarize your home because you weren't home, they could have done it for years now.
I happily live in this world of openness and enjoy the benefits these technologies have given us. I'm just drawing a line as to how much data/privacy I am willing to hand over.
Suspicion for Those Without Accounts
Those who think that not having facebook account makes you suspicious are mostly looking for link bait. There's no concrete evidence that suggests people are more connected, healthier or more social than those who choose not to use social media.
I've never had an prospective employer ask why I don't have a Facebook account. LinkedIn seems to be good enough for most companies.
There Are Better Alternatives
I'm a huge fan of Twitter because I find it more useful and less time-consuming. Even now, a quick scan of TweetBot can lead to a full Pomodoro cycle of scrolling and Instapaper links.
In the age of digital maturity, I have found niche websites like Meetup, Forrst or Untappd to be more engaging and encouraging social interaction.