The more we communicate electronically, the more we are identified by name, by handle, by symbol, or another identifying character. And, from there, more often than ever before, we are communicating via a mark of some sort. As I harp on it, time and again, from a legal standpoint a mark is an indicator of source. It is an identifier of who or what provided goods or services. And services include providing information and communication to others. That means each of us who is online has a mark of sorts in the online electronic world.
So, where your face or voice in an in-person interaction will surely identify you as the source to others on the receiving end of that in-person communication, in the online world your handle or other identifier will do that for you. In the off line world, others can have the same name as you, but online if someone were to have your same twitter handle or e-mail address as their own, we would call that hijacking, squatting, hacking, or stealing. Someone can look just like you in the off line world, but they cannot have your same name online; too much confusion would ensue. With highly similar online names and addresses, such confusion already happens. But you can choose your online mark-and in that space communication, connection, and information reign supreme. Tone and delivery are the online body language. That being the case, in the connected electronic world, where marks do much of the talking and representing, indicators of source are more important than ever.
As for how this fits under the law and connects with trademarks, service marks, collective marks, etc., you'd want to consider whether and how a particular mark has been used in commerce. That's a topic for continued thought and discussion...and special analysis based on your use, intended use, and overall plan...because, like the off line world, use online and even in social media contexts can take on different forms: sometimes it's business, sometimes it's personal, and sometimes it's a little of both.