First of all, the 400k is a number, not a metric. Techcrunch only mentions it to create a polite segue to their skepticism. Bear in mind that the number of use

First of all, the 400k is a number, not a metric. Techcrunch only mentions it to create a polite segue to their skepticism. Bear in mind that the number of users is particularly useless, because about.me is an vanity URL-based service. People stampede to reserve a /<firstname> domain. This is also the case for Twitter, Tumblr, and the Facebook vanity URL, or whatever they call it.