Twitter is an interesting phenomenon, and its evolution is one of its more interesting aspects.
Initially Twitter was essentially generic version of the Facebook Friend Feed - great if you're eighteen and think the world revolves around you and your mates, but not so great as a communication medium. Most of the content was fairly inane (just like Facebook), and disjointed. To be sure there were amusing people such as Stephen Fry to follow, and some interesting ideas such as Cry for Byzantium, but depth was generally lacking.
The use of hashtags allowed the construction of folksonomies, such that if you were interested in the medieval period you could include #medieval in your posts. Now we could argue about early versus late, high middle ages, whether we mean 476-1453, when the renaissance became truly distinct etc etc, but basically everything posted would be medieval - it reduced the noise.
And as a folksonomy we can make it up as we go along, making readily guessable tags such as #viking, #anglosaxon, and perhaps less guessable ones such as #britannia.
And as you use the search you start to find people who you wish to follow, as they post regularly on related topics. You can now create a list to allow people who follow you to follow the people you follow - possibly a bit incestuous and circular, but it means that your feed starts having crowd sourced characteristics, so if one of the people you are following misses something there's a good chance you'll pick it up anyway by someone else on his or her lists.
For example, let's say I see a blog posting about the Jorvik viking festival next year. Despite regularly tweeting with #viking I don't because (a) I used to live in York and (b) I don't now and (c) won't be visiting when the festival is on (all true). However I also have someone called Pete on my list who doesn't have my hangups about that and who posts the link. And maybe I have someone else called Karen on my list who posts a different but related link. That way the message still gets through in a structured and sensible way.
Of course I'm ignoring the role of reputation. All of us #medieval users have to assess the worth of someone's postings before adding them to the list, just as fifteen years ago with usenet news we wrote complex kill files to filter out the known loonies and people with fixations about leather underwear.
But in a sense reputation is implicit - if the quality of the information I post is good, I have a reputation and am thought to be 'serious' - and so very gradually it turns into a community of people sharing common interests - a social network by any other name.