According to 25H "all parses" over last monk it seems that we have:
Priest (18.85% disc, 9.05% holy)
Shaman (21.68%)
Paladin (20.74%)
Druid (17.11%)
Monk (12.56%)
Monk will typically be the lowest as their class rep is lowest since they are new and not super-OP in the healing department. There is a certain pitfall to combining disc and holy, because you can't really separate them as many priests do switch (even from fight to fight) but it would also be wrong to just combine them as many others will consider themselves primarily one of the two specs. Suffice it to say I would put Disc Priest representation higher than 18.85% (accounting for those who flipped Holy some places) but lower than 27.90% (which would double count "mainly Holy" priests).
Consider also that when it comes to DPS class/spec representation often classes with multiple specs that do the same role (for instance pure classes, arms/fury, frost/UH, basically any two DPS specs except separate melee and ranged ones) are often lumped into the same population for similar reasons.
We've definitely dipped and I think part of the reason it hasn't changed more than already is just because of the sheer difficulty of maintaining multiple top-geared/legendary/ready raiding alts.