this didn't bother me either, until MIL commented one day that she liked the blue dungarees i'd bought DC
now i'd only bought them about 3 days earlier, and not spoken/seen her since, until I realised my SIL had uploaded a picture of her DC and mine together as her FB picture
i didn't mind it... BUT that was the start of little glimpses into how much MIL is on FB, and it makes me uncomfortable about how much time she spends checking up on what people are doing/etc
for example, one of DH's brothers regularly posts on the move (from his phone), there's something a bit disturbing about MIL constantly checking what he's up to, commenting on where he is, wondering who he's with at this or that cafe. it just freaks me out that someone knows what's going on in our lives without the chance to catch up with them for real when we meet up, or pop over after school/etc
So. YANBU. it would make me uncomfortable too, and it's one of the reasons why I asked SIL to take down the picture and stop posting them completely publicly without permission. i have no idea who's on her friends list, i'm sure my children aren't interesting to random strangers, so why do they need to have them plastered across the internet?
Besides, DH has spoken about how FB / Google are looking into facial recognition software, it sounds crazy (but he works in IT, programmer, so trustworthy source of tech news!) AND i would hate to think that in 20 years DC goes to uni, someone googles his name, and BING 10, 000 pictures of him as a 5 year old pop up because his auntie has been posting up his picture in public.... (although the name would be a givaway, d'uh, but you know what I mean).
SO, YANBU. anyone who plays the "stop being so paranoid" card really hasn't thought this through, really.