[cabbageicefruit]

Tumblr is one website. Not 500,000. Even though 500,000 people have a blog there, it is still tumblr.com. Even if you do set up a subdomain such as my-blog.tumblr.com it still just redirects to tumblr.com/my-blog. Counting tumblr as 500,000 websites is like counting everyone’s Facebook profile as its own website.

[evanelias]

The relevant number is 500,000,000, which is 1000x larger than the number you're citing. But also your comment about subdomains is wrong: redirecting to tumblr.com/my-blog is an optional setting, with the default being to use a subdomain. You can also use custom domains rather than a tumblr.com subdomain. Tumblr has had this functionality for almost its entire existence.

[chiefalchemist]

Half a billion is 500 million.

[chiefalchemist]

I'm fairly confident that's not how WP.com does it. On WP.com - via WP multisite - each site has it's own instance of WP and therefore each counts as a site running WP.

Tumblr's injection would result in a significant increase in the number of websites running WP.