I don't have a source, because most of the young kids whose lives have been shared without their consent have been having their lives shared for less than 10 years (Facebook started in 2004 and was rolled out to the general public in 2006 [1]), so we'll have to wait before academic studies start coming out on the topic of oversharing.
I am coming at the topic anecdotally (where sites like STFUParents[2] exist for people who are tired of overshare or people who have been fired or arrested for things they shared on social networks come into play.)
If anyone has links that either prove or disprove that sharing pictures of small children/sonograms is beneficial, I would be really interested in seeing them.
I just tend to skew towards guarded when it comes to sharing private medical information and pictures, especially of minors who don't have a say in whether you do or not.
[1]https://www.facebook.com/notes/facebook/welcome-to-facebook-...
[2]http://www.washingtonpost.com/blogs/on-parenting/post/stfu-p...