Top 10 US web hosts ratings, based on domains, December 2011.

Rank Hosting Company Domains
1 Wildwestdomains 31,863,237
2 Oneandone 2,887,127
3 Hostgator 1,936,148
4 Yahoo hosting 1,711,911
5 Bluehost 1,540,233
6 Dreamhost 1,195,889
7 Hostmonster 764,249
8 Mediatemple 457,489
9 ATT-hosting 428,846
10 Rackspace 402,756

The entire data set currently consists of all COM, NET, ORG, BIZ, INFO Domain Names.

The clients of a Web Hosting company are identified as all Domains served by the nameservers of that particular Company.

Source of data: webhosting.info