So I took this 3-4 year break from fandom (every fandom, all the fandoms; it was odd) and now I'm sort of poking my way back curiously. And everyone is now on Tumblr, and pinboard, and livejournal seems like somewhat of a ghost town. What happened? Why Tumblr?
Oh, wow, well—first off, welcome back! :D Secondly, OH WOW, WELCOME BACK. What you’re asking is super-complicated. Basically, as you’ve noticed, U.S. LJ at this point is ONTD and its Asian fandom spinoffs, GRRM, and a few fandom stragglers that refuse to leave or occasionally need to step away from Tumblr to do things Tumblr can’t do. And obviously Delicious as you once knew it doesn’t even exist anymore.
The reasons why boil down to a bunch of different factors that all came to a head between 2010 and 2012. If you want the really long version, in 2012, when we were all still very much in the thick of it, I actually wrote multiple articles detailing what happened to LiveJournal. If you’re in the mood to read them they’re here and here (this one also talks about the Delicious –> Pinboard exodus).
Otherwise, this gets a bit long, so I’m going to put the full answer under a cut.
The short answer: we were driven here by the Russians, Yahoo (ironically), and Harry Styles.