[caspianm]

On Twitter and Tumblr you can make extra accounts to participate in discussions you're interested in, and select people to follow based on that, so the feed system is okay for talking about things other than yourself if the feeds don't include everyone you know by default.

Tumblr has some pretty good discussion about movies and books.

Twitter not so good for discussion because off the length limit, but there's plenty of people posting concise observations and jokes rather than posting about themselves.

On both systems, people can reply to content from strangers, and there's lots of conflict arising from that.

I do think Tumblr would be improved by making it easier to have discussions that don't go to all your followers by default, for example like on Twitter where if you tag people at the start of your tweet, it doesn't go into the main feed for your followers who aren't tagged.

Or you can go all the way to partitioning a system into topics, as with Reddit. I wouldn't call that a social network though, you don't just casually start a conversation with people you've chosen to connect with, you start a conversation with a subreddit.