Yup... LinkedIn was the social network I didn't use. For what it's worth, their Cancel My Account option was relatively easy to find and relatively painless to navigate.
Maybe the ease with which one can cancel an account is at least partially a factor in inflated membership metrics? According to stats here, an average user spends 15+ hours a month "on Facebook." Based on the fact that some high school students I know are logged in 24 hours a day, there are a lot of unused accounts sitting on their servers. I know mine is used less and less with the advent of Google+.
Have you actually tried to get service at a Genius Bar recently? The last two times I've been in to have someone look at my MacBook Pro (once for the infamous "failing keyboard" issue, and today for the "charged batteries don't power the machine" fiasco), there have been ZERO appointments available for the entire day at both (southern California) outlets within driving distance. In northern California, the Emeryville store Genius Bars are fully booked DAYS in advance.
While I'm occasionally accused of being a fanboy, I've been very disappointed with how difficult it is to get basic care for manufacturer-defective equipment at the Apple stores. Maybe all that "retail traffic" is simply people waiting around to get their defective machines looked at?