[Tyrannosaurs]

Posterous was a year later than Tumblr but first to market doesn't count for as much as some people make out.

For social sites (Twitter, Facebook etc.) it's important because of the network effects but to read and interact with a blog on a particular site you don't need to have a blog on that site yourself (else we'd all still have LiveJournals). That means age or growth doesn't mean too much.

Of course that counts for Tumblr too which means that you've got to look at more standard factors for growth - functionality, cost, brand awareness and so on - and in these areas Tumblr seems to have an advantage which I suspect is why it's growing faster.