Looking at one of the videos on justin.tv, it appears there is a +20% XP boost for playing online (rather than just against bots) and a further 15% XP boost for completing a match.
I also noticed that you get around 500-700 for being on the winning side.
Average XP for a decent player seems to be around 4,000 XP/match which lasts around 10-15 minutes. So it should take about 75 of these 10-15 min matches to get to level 20 (300,000 XP), or 15-20 hours of gameplay. Feels like good numbers to me.



