http://www.dongleauth.info/ maintains a list of sites supporting U2F devices. There aren't many yet, but it looks like a Wordpress plugin exists that may end up in core. There are also a handful of additional sites listed in issues and pull requests on the site's GitHub page (https://github.com/Nitrokey/dongleauth).
(I'm not affiliated with the site, I just check it periodically and thought it might be of use here.)
That's new! Took more tries than normal to register, but that's great. They took so long to roll out even TOTP based auth, I figured it would be ages before seeing U2F.
