Similar to a photo-op, which is a opportunity to take a meaningful or memorable photo, a Facebook-Op would occur when one takes a photo just to upload it to Facebook later.
Friend 1: Why did you take a picture of all your schoolbooks laying on your bed and your laptop open? Are you having some sort of all night study sesh?
Friend 2: Nah, braaaaaw. We just took that for a Facebook-Opp. Makes us look like we were studying all night, we were actually getting shwasted!!!