Tumblr can be very powerful.
But just like the above mentioned methods, if used wrong it is just annoying to the users and pointless for you.
The momentum must come from genuine interest not forced by incentives or wild adding of likes, friends or followers.
You have 30.000 followers? Great! How many give a toss about your tweets?
20.000 likes for your Facebook page? Great! Bought from a broker, and operated by indians?
Sorry got a bit carried away
But yes tumblr if it matches your product and user demographics