Most social networks would see rewarding early adopters as a benefit rather than a drawback. Besides which, TheThreeWordName or JohnBSmith1982 is still a lot more memorable than a random alphanumeric string.

As another person said further down, it's why we have domain names rather than IP addresses. Sure, the way in which they're distributed might be suboptimal, but not nearly as suboptimal as making everyone have to remember the IP address or rely entirely upon a search function that returns ever-changing results.