Tumblr is a micro-blogging site. Micro-blogging is just smaller, both in content and file size, from regular blogging. So it’s more about posting things that can be consumed in under 3 minutes (max), normally a matter of seconds. My favorite Tumblr is The Daily What, which I think gives a really good idea of what you can do with a tumblr. They do have tons of followers, and as such, companies advertise with them, so it earns them money. I don’t know if it’s enough money for that blog to be the owner/s only job, but it’s still money.
A key thing to notice about Tumblr is that there’s very little interaction. You don’t post something, and then have a thousand comments discussing the matter. If you have any comments at all, it’s usually something short and a bit of a non-starter like “LOL4RL”. Instead, people “reblog” your post on their tumblr, so that it gets passed around quite a bit.
I think that we get more Tumblr spam than other blogs for a couple reasons: One, I think it’s the same mentality that makes people try to have 4 thousand friends, 3,938 of which they’ve never even met once on Facebook or Myspace. It just turns into a popularity contest. Two, you don’t have to put as much work into a Tumblr blog as you do into each post of a different blog (or, at least, it appears that way). There aren’t a whole lot of posts deconstructing the latests speech from that up-and-coming governor, or recipes in which someone has painstakingly made a dish a few times to perfect it and taken pictures of the beautiful gourmet layout. It’s more “Hey, look at this cute video of a cat I found” or “Hey, look at this amazing stand-alone quote I found” or “Hey, learn about an important news story in 15 seconds”. So it appeals to people who are looking to make a quick and easy buck, and be popular in the mean time.