I disagree and find it to be illogical, as Tuvok from Voyager would put it. xD There are many individuals who indeed lack confidence, however, they still display an image of themselves for all to see. *Tuvok voice*

I also can't help but to think this is a little arbitrary. I'm sure there are people here who have kids, and perhaps stick pictures of them on their Facebook, but do not feel as though they are living a full life. To be fair, neither of us can say for sure.

I'm actually more inclined to agree with you on this one. I always get the feeling couples are showing off when they litter their Facebook and what-not with pictures of them together, typically kissing, smiling, hugging, etc. I'm guilty of this, myself. xD

There could be any number of reasons for not showing an image of themselves, or rather, showing an image of something like scenery, but I suppose it sounds plausible.

I'd personally go with them wanting to show that they are social; that they are not alone; that they matter. If I had such a picture of that as my Facebook avatar, it would honestly, most likely be for that purpose, albeit sub-consciously; when I think about it, it makes sense, at least for me. They might even do that to look more appealing to the opposite sex, especially if they're hanging around with the same gender as the one they wish to attract. (adjust as necessary for different sexualities)