I don't exaggerate when I say this: twitter has made me smarter. If I'm interested in a new field, I just follow the experts in that field that are on twitter. The conversations and the discussions not only make me feel like an insider, but make me explore the field in a much deeper level. Three of the fields that I have gotten 'into' because of twitter are Urban 'renewal' sort of projects (citylab, atlantic cities, etcetera), the book reviews circle, and a certain subfield of computer science I won't mention, because I'd probably be the only intersection of those fields. : P
Sometimes, some people I follow tweet things I'd rather not hear. So I simply mute them. Done. (This is however NOT a apology for all the awful harassment that does happen)
I cull my 'following' list to get to 300 people once every couple of months, so it doesn't get out of hand, and it's worked perfectly for me. I can catch up with pretty much everything that appears on my timeline. I Like twitter because it doesn't 'curate' my content for me. The day it decides to get rid of the 'everything' timeline will be the day the 'decay' begins.
Perhaps my viewpoint is tainted, but in the past six years (that's how long I've been on it/using it), the number of twitter users has been growing (at least in terms of people I know), and their quality increasing. I realize harassment is still a huge issue, but despite that, Twitter is still a great community : )