Matchstick
Adventurer
But, the question has to be asked, why are the numbers small? There are thousands and thousands of people using Virtual Tabletops right now. There IS an audience for VTT's and WOTC has the advantage of being able to drop a VTT that is tailor made for it's flagship game, something no other VTT can do.
And what did we get? A glorified chat client, die roller with virtually no macro ability, and a whiteboard that you cannot actually import images to.
Tabletop Forge had all of those things in probably less than a month of development. They started around the end of January and look where they are now. I'm not sure what the WOTC people were doing.
That's a good start. But what they needed was a "Summon Party!" button on the VTT, that a player could press at any time, whereupon they'd be instantly connected to the other 50 or so other DDI subscribers who were also online and also looking for a party.
Because here's the thing: there are better VTTs out there, but if I want to play a game on Google+, or using Maptools, or Fantasy Grounds, or whatever, then I need to be in touch with the individuals running the game and we need to coordinate a specific start time, then all log on at once. If I find myself with a Wednesday evening unexpectedly free, chances are that I can't game. Provide that "Summon Party!" button, and you close that gap, and potentially provide people with a service that they didn't know they always wanted.
I think Google+ and Tabletop Forge are trying to do that kind of thing with Hangouts. You start a Hangout with your game going and people that want to play can join. Create circles of FATE gamers, or D&D Next gamers, or whatever, and then just start your game and let everyone in the circle know.
It's not something I've done, but I think something like what you are talking about would be possible on G+, which seems to have a TON of active RPG gaming and gamer activity.