"w" is short for "warai," which means "laugh." The w's are Japan's equivalent of "LOL." The more w's there are, the funnier the thing is to that person.
"How about them apples? w" versus "That was hilarious! wwwwwww"
If you look at Nico Nico Douga videos, you'll see a lot of comments full of these w's at the end.
It can be hard to tell, sometimes, a Japanese player from a western Japanophile though. But from what I see during Splatfests, it's not that common for someone to put a Japanese name as their handle, especially not ones like "Tomoya" or "Yusuke" where there aren't many anime or game characters with that name but it's fairly common in real life. And then there is the Japanese Americanophile somewhere in between, which I'm sure comes up from time to time too. It's a niche group, Japan's equivalent of the hardcore anime fans here, who can name you the last ten TV voice roles Jennifer Hale has done or the entire cast of Fish Police.