I suddenly realize I haven't plugged hubski on this place yet. Hubski is one of many link aggregation/discussion sites on the Web, but it has the distinguishing feature of people on there being somewhat mature and educated when trying to discuss something. I've been a member since about two years (under a username I'm keeping secret for now), and so far my stay has been rather pleasant.
Yes, it is a centralized service. But as centralized services go it's a whole lot better than say reddit, a site which has only recently realized that perhaps hosting a platform for neonazis and virulent fat shamers is perhaps not the best thing to do..
I should also note here that some of the features that hubski implements are things that GNU Social/OStatus has had support for for years, but has yet to catch on in a major way. Neither has hubski, of course. The future will show which one ends up being more popular..