The sustainable growth of Pinterest is fascinating to the analytical aspect of my brain, but the common sense, practical firefighter part of my brain struggles with why it exists in the first place. I see people sick people at shelters browsing Facebook (on their phones/tablets via wifi) and literally cannot believe my eyes.
I have signed up and participated on all major social media sites, but have ultimately left them behind. My life wasn't drastically improved while using Facebook, Pinterest or Twitter. I have read more books and engaged in lively (albeit slower and more thoughtful) conversations via email since disavowing their use.
I guess my point is that social media, in its current incarnation, hasn't cemented utility beyond advertising. Most of these sites tend to afflict one with or are used because of ennui, in my opinion.