I've been sort of bemusedly watching my Twitter feed and the hashtags that appear on it. I've been vaguely bothered by the whole hashtag phenomenon. On the one hand, it seems like a great technical solution to the problem of how to find out what people are saying on a particular subject. On the other hand, often (but certainly not always) people are using hashtags more as a trivially easy way to show their agreement with some expressed concern, like Michelle Obama and her #bringbackourgirls sign. In a way, they're like the MIA/POW wristbands so popular in the '70s, or the peace sign pendants in the '60s. While at first they may have been a meaningful symbol of where someone stood, they quickly became meaningless and insignificant tokens used by someone who really doesn't care (or even understand) the issue at hand, but wants to pretend that they do.
Why would someone do this? Social acceptance is the obvious answer, and I see no reason to doubt this is the major motivator. One variant of that is a bit earthier, but memorable – I remember overhearing a conversation between two boys in my 12th grade English class. The gist of the conversation was that if you wanted to get laid by a hippy girl, the key was to wear a peace sign necklace – just 99 cents at a local store. Cheap, easy social acceptance at work :)