I Quit Facebook and Nobody Noticed
Not even my wife. It took her four days to notice I wasn't on Facebook anymore. It makes sense, in a way. The way you engage with Facebook (or G+ or Twitter, for that matter) is just a feed populated by updates from your friends and the pages you follow. When I left, it just meant that my friends were missing a couple of posts a day from me. That's not something you tend to notice immediately.
It's weird, though. At some point, you'd hope a few people would sit up and wonder "Hey, where's Sam?" Two weeks after deleting my account, I think four people have noticed.
The upside is that I'm much more motivated to get out of my office and "engage" with my "friends" offline. It's sort of like using a muscle I nearly forgot I had.