I'd say it's not very active, but it's never taken me 15 minutes to get a match. At most like three, though I do end up running into the same people a lot, even though I've got my settings to accept pretty much anyone.
I think it's very fortunate that players using the PS3 and the Vita can play each other.
Still, the game can be found cheaply, and the single player offers quite a bit. If you got it, you'd be another player out there, and that's always good.
You can find the most players by setting your filters to ANY. Also, if you're patient enough, go online at different times of the day to find out when most people are playing. It took me a while but I kept at it because I really wanted to unlock all the rare bikinis. It should be easier to find people to play with now that Vita users are also playing this game online.
Im a huge fan of fighting games and with my newly bought vita i already have ultimate marvel, playstation all stars and street fighter x tekken so im still not sure if i want this or ninja gaiden 2. Both ninja gaiden and dead or alive are good but ive heard ninja gaiden has huge frame rate issues and this game online isnt active so i cant decide.