Maybe I'm completely wrong but to me it always felt like Tumblr landed on a seriously awesome name. There's just something nice about it: simple, clean, clever, cute, and very contemporary. It just feels nice. I wouldn't be surprised if Tumblr's name played a small factor in getting people to come check it out.

Sadly I think posterous's name is quite bad. It sounds like a developer came up with it (no offense guys), it doesn't roll off the tongue and it sounds intimidating.

These are just my hunches, no data at all to back any of this up, but I do feel like the right name can make a difference.