Before jumping to any conclusions, you should contact the editor of the online magazine and ask him about it. Someone may have submitted the story with your byline, hoping to make money off your name--if indeed this online magazine pays much of anything. If it doesn't, then what would be the motive?
This an odd thing all around. You say you haven't submitted to online magazines, so how did that magazine get your story? For the editor to steal it, he would have had to have access to it. Therefore, the only logical scenario is that someone else presented him with the story, using your name. This person would likely have had to e-mail the story to the editor, and might have put his address in the e-mail. The sole reason would have to be money, since the writer/thief gains no reputation points from a story published under your name. But how much does this online magazine pay? Few of them pay anything that would make such a theft worth it.
Also, are you a writer of such reputation that it would make someone want to use your name? If not, that further makes the motivation for money unlikely. Was the writer/thief paid for the story? (You can find that out from the editor of the ezine.) If so, that might be a means to track him down--whether by address or online payment, unless he has managed to conceal that as well. Very elaborate indeed, if such is the case.
Just some thoughts from a short fiction writer.