I think Tumblr can be useful for a single rec, because it fits well with one of their post-types (the link plus description). Overall, I find I'm using Pinboard more to track fic and see what others are rec'ing these days, but I've found a couple of pairing-based Tumblr's really useful for finding out about particular stories for smaller fandoms.

One thing I really enjoy using Tumblr for are quotes from fic. Whenever I come across a really great line, I tend to put it in Tumblr. That's a kind of rec, but I think its also an example of how the Tumblr design/interface is really more set up for certain things like images, quotes, and links... and less for actual humans having actual conversations with each other about fic. (Which, as much as I'm a confirmed Tumblr user, is the thing I most dislike about it.)