That's really not an issue. It isn't much easier to remember that I'm "John28161" on a busy site. Websites have been offering "I forgot my username" functionality for ages, so as long as you remember your e-mail address, you're fine. Also, just let users bookmark their personal profile page (foo.site.com/user/83755567565) easily and it's solved even if cookies are deleted. Apps won't have a problem either way.