Completely false. Think of email. I've been told by people running large-ish, open signup websites that Gmail, Yahoo! Mail and Hotmail together account for more than 70% of users. But, so what? Three choices is already better than one, and then you have a long tail of a zillion other email providers making up the last 30%, which you can use if you don't like those 3, and finally, if there's no third-party email service you want to use, you can run your own server.
With Facebook and Twitter, you don't have that choice. If you want to connect with friends on those services, there's only one provider available, and that's it. Tent will give you that choice back — even if, in practice, a few large servers have the most market share.
There won't be a million servers, most people will cluster around the few most popular providers.