I think it can be both. I'm using Tumblr as a blogging platform than a social network. I spend more time writing content than looking the posts of other. I migrated from Blogger to Tumblr coz I feel that blogging is easier there.
I have 12 blogs on Tumblr (and counting), almost all of which I post to frequently - and the only thing I ever use the dashboard for is to make those posts. The only social action I have ever taken was to reblog a recent post I found on reddit, mostly just to see what would happen.