Depends where on FB you're asking, and the reasons. Asking "can anyone watch the kids on Saturday?" on your own status is alright, as is "Looking for a regular babysitter to work every Friday from 7-10, $10 p/h, call [number] for an interview" on a local page. But posting the first message on a local page (so asking a bunch of strangers to look after your kids, no screening beforehand) is a bit dodgy...