[webwright]

There's a timeshifting problem at work here. If Tumblr was indeed first to market, then it SHOULD be ahead. They are similar products with similar (viral) growth styles. A more interesting questions would be this: Is Posterous growing as fast as Tumblr when Tumblr was at this stage/age.

Similar articles were written about Facebook and Twitter and now Twitter is getting 370k new accounts per day. It just takes time.

Personally, I find the social network features of Tumblr really jumbled and confusing as an outsider. LiveJournal was a social network, too, and it plateaued.

[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.