I'd been rather absent from LJ for the last two years and more active on tumblr but looking back at these two years, I find that LJ has much more to offer than tumblr ever will. Most importantly the connections one makes.
On tumblr, it's difficult to have long conversations - you're restricted to messages and it's a hassle to follow them without losing the overview. Another thing is that you rarely get to know people, you're following a bunch because of their graphics/fanfiction/etc. but who they are, what they do or anything personal is A) lost in the abyss of the tumblr-network or B) never posted by people because they feel tumblr isn't exactly a place to share your innermost thoughts.
Here on LJ, the familiar atmosphere is the first thing you notice and in my opinion people find it much easier to share their daily thoughts. A post can have hundreds of comments and each of them can spark a debate/hours of conversation/etc. and yet it's still easy to follow them. If you make friends, you know so much about them after just a few days and you find you have so m uch in common with them that fandom becomes sometimes a second thought and their daily life takes lead.
Back in the day tumblr was the new IT but now I find it only useful for graphics and some fangirling. Especially for the new shows it's a good place to get your daily dose of gif-sets/picspams/etc. and I'm still going to be on tumblr but I've also decided to come back to LJ. It's the first place I ever joined and I found so many awesome friends here.
Home is where the heart is, and mine is definitely here.