There are probably as many people playing DotA as there is normal warcraft. Why is DotA so popular? Well I think it is fairly obvious, it’s mini-WoW with the ability to play all sorts of different characters. There is the same grinding of creeps, teamwork to tackle harder challenges (Roshan == instance?), PVPing, items, and inventory. In fact, I’ve seen people playing DotA and leaving mid game because the WoW server is back up.
Of course, DotA is $15/mth cheaper than playing WoW but I think there is still the same waste of time.