For every couple airing their dirty laundry, there are a half dozen who seem unnaturally preoccupied with presenting us with an idealized view of their lives. In some sense, Facebook acts as a virtual highlight reel for all of us, but these couples seem to share only gloat-worthy updates whose sole purpose seems to be inspiring jealously.
You reveal as much in what you share as what you don't. Your status updates about how your girl is "the greatest in the world" start to seem smug when you add "don't be jealous" or "she's better than yours." Likewise for the couple who is live-posting their vacation pictures, boasting every three minutes about whatever they're doing that is so much better than what you're doing.
It's not that we're jealous because we're at home eating pizza bagels (although, we probably are), but after your 100th declaration about how amazing your life/significant other/vacation/new home is, we start to wonder what's working beneath it all and who the intended audience is. Your ex? Your single friends? Her ex? Most of the time, it's easy to tell who you're aiming at, and even easier to discern who isn't interested in your posturing—us, and probably 98% of the people on your friends list.