From RootDig.com, Michael John Neill’s Genealogy Website:
Google That DNA Username
Individuals who set up internet accounts often use the same username on multiple sites.
If a DNA test submitter has no tree and does not respond to messages and you can’t figure out who they are, try a Google search on their username. That’s helped find quite a few people who had profiles or accounts on other sites–Facebook, MySpace, etc. Sometimes those profiles or information on those websites is enough to determine who the person is or provides another way to contact the person.