ISPs, plausibly. But that's because physical infrastructure is such an important component. It's the same reason you generally only have a single choice for an ISP. THAT's the problem there, and that's why there's a good argument for ISPs to be utilities, and why net neutrality is so important.

Hosting and domain registration are commoditized services. If one doesn't want to do business with you (or vice versa) there are thousands of other options.