Maybe this is just my personal bias, but older folks seem more sensitive about online slights than younger ones. Remember those older gals who almost broke up over one of them "flirting" with the other's ex? Also, one time my mom dreamt that my brother had a serious head wound, and then he didn't e-mail or call her back in, like, 48 hours. I was eventually forced to intervene on my mother's behalf, so I texted, "Call mom. She dreamt you had a gaping hole in your face and now she's worried."
The moral of that story is we shouldn't e-gnore those of the Groovy Generation and beyond. Therefore, defriending Mr. and Mrs. Rosenblatt might result in a hurt follow-up e-mail where they ask you awkward questions about what they did to deserve this. Was it the inspirational-quote fridge magnet they got you for your birthday? The time they e-mooned you via holiday elf? Presumably, you want to avoid this.
Of course, it also depends on what kind of relationship you have with your ex's immediate family. My ex is still Facebook friends with my parents. My parents are even friends with my ex's sister, even though they've never met and probably never will. I, for some reason, don't find this weird at all, though I do find it weird that I'm still friends with my ex's old roommate's cat, who won't shut up about the "elusive laser pointer" anyway. I'm also not friends with anyone else's parents except my own, but that's mostly because I don't want to offend anyone's meemaw with my links to smutty Muppet videos. My own parents are already quite used to this, so there's no point in hiding from them, as much as they perhaps don't feel the same way. Although I've definitely seen my fair share of pixilated elf ass courtesy of my folks. So, you know, nobody wins?
If you plan on maintaining some kind of relationship with your ex's family, even in a limited capacity, then keep them as friends, but create a separate friend category for them and hide updates/photos you think might not go over so well or that you don't want your ex to be privy to. This should take care of your censorship woes and give you a better grip on your privacy in general.
As Miss Piggy said, c/o Peaches in "F*ck the Pain Away," "Huh? What? Right. Uhh."
P.S. Here's a refresher on handing break ups gracefully in the digital age.Follow us on Twitter: @annapulley and @ExhibitionistSF or Facebook