DH and I were just talking about this the other day. I don't necessarily think you shouldn't put your scan photo on FB, but I personally wouldn't. I have a lot of friends who have used their scan photos as their profile photos (which anyone can see, even if you've set your other photos and info to be private), but these were generally people whom IMO overshare on FB.
I guess it depends on what you use FB for. I have a mix of family members, friends, and professional contacts as FB friends, and although I do post some personal details, I never post anything that I wouldn't share over a cuppa in the office. Even though I'm very close to many of my FB friends, I don't think my boss - and my grandmother! - need to hear about my hangover, for example.
I also don't think that most people really need or want to see a photo of my uterus! I've really only shared my scan photos with family and a few close friends, and have scanned them in and e-mailed them to people who live very far away. But to each her own!
In any case, my latest scan photo is quite blurry as my baby wouldn't hold still, and he looks a bit like Skeletor, as scan tech was in a hurry and not that bothered about getting a good photo. It would definitely not make a good profile photo! He was very cute in his first scan photo, but that was way before I was ready to share my pregnancy on FB.