I am tired of "social media experts" wondering why Twitter hasn't taken over the world yet. It's because, duh, we're not all "social media experts," and that's the sort of folks it works best for.
Twitter works for (a) people who others tend to search out, primarily celebrities and (b) people who want to be in category a and can develop a niche or hook. So I want to keep up with Neil Gaiman; I follow him on Twitter. I doubt he follows me in exchange.
But Twitter doesn't make it easy to connect with friends or acquaintances, not like Facebook does. I have a friend I know is on Twitter, but I don't know her username. I searched for her using her real name, but no luck. I have other friends that I know are on Twitter, whose usernames I know, whose Tweets I don't read because on Twitter they are an expert - on something I don't care about. They tweet about relational databases or something instead of their life. I care as little about their expertise as they care about mine.
Sure, there are services that help you find people of interest. In my experience, 95% of the suggestions are garbage. Of the rest, some are theoretically interesting but not practically so. I'm interested in higher education news, but if all you do is tweet articles from the two main industry pubs (which I already read), what value do you add? (Especially when you make me go through your website to get to the actual article.)
The best use I've found for Twitter is backchanneling at conferences and events, yet in my field there still isn't a critical mass for this to really be valuable.
The thing is, Twitter seems to be best for disseminating information or hive-mind commentary. It isn't a relationship/network manager like Facebook and other social media websites, which is really of interest to more people on a daily basis.