I've been trying to get a lobste.rs invite for years! It's a tough community to get into. I hadn't heard of Tildes before, but it looks cool, too.

Reddit reached Eternal September a while ago, so the key is to find small (yet active) communities whose content doesn't make it to r/all. I moderate a cycling community there, but we're really strict about what type of content gets posted. It sucks that we as a mod team have to do that, but if we don't our community gets overrun with memes, image macros, and rage comics.