This is a big part of why I stopped using Twitter. Of course, every tool has its uses and misuses, and Twitter can be an excellent way of discovering new content from favorite producers, but using it myself turned into a popularity contest.
PG writes about popularity contests in Hackers and Painters, and how geeks prefer doing and learning over winning popularity standing. I didn't read H&P until after I had quit Twitter (and largely quit Facebook. Facebook is nothing more now than "email from my mother"). But that resonates well with me. I found myself tracking trending hash tags and trying to come up with witty, pithy tweets that I could also hashtag in kind. I had specific strategies that I would test and track the retweets and replies. It was kind of sick, looking back at it now.
But looking back at my internet life, it wasn't always like that. If you consider the BBS and web forum to be proto-social networks, this type of behavior (in certain communities) was neither broached nor tolerated. A much greater emphasis on discourse existed. In certain online communities, posting "+1" or "first" (or its more recent analogs "this" or "feels") was a fast track to banning. But those networks had something that Facebook and its ilk lack: administrative moderation, either by a staff of people or by community members with elevated privileges.
They were also geared towards the long form of prose, rather than the pithy-saying style. Twitter still has it's 140 character limit; it is considered the culture of Twitter. Facebook for many people functionally had the same limit as they interfaced with it through MMS. I don't know if they still do now, but at one time Facebook had a character limit on status updates and replies; even if those limits were removed now, the vast majority of Facebook's users are trained towards them now, and we see a new article every week talking about Facebook being unable to attract "new, teenage" users. Similarly, Tumblr's easiest levels of contribution are the reblog and photoshares.
When the discourse is so severely limited, then there can't be a discourse. People will revert to what is easiest: posting things that are not meant to engender discourse. People brag, always have, it's natural. But the signal to noise ratio is much worse now because services like Twitter, Tumblr, and Facebook make the noise so much easier to create than the signal.
EDIT: sorry, typos, still on my first cup this morning.