In order from most common to least common DPS: BLM, BRD, DRG, MNK, SMN.

If I had to really give average and arbitrary numbers to it, I'd say that out of a pack of 10 DPS, 3 are BLM, 3 are BRD, 2 are DRG, 1 is MNK, and 1 is a SCH that was a SMN but couldn't get a party.

As others have mentioned, I'm going to assume that NIN will take top slot for a while.

Tanks, I probably see 3 PLD for every 1 WAR. Healers, I probably see 2 WHM for every 1 SCH.

Note - that's my observations on my server - and in the DF. Your mileage may vary.