Ok, let's look for some data. Over the years, there have been several platforms for 2p dominion online, and each one had its own stat-gathering site. Correct me if I'm wrong:
BSW
isotropic - councilroom
Goko/MF - Salvager
MF 2.0 - Salvager for 2.0
SI - Scavenger
I'm a bit fuzzy about the dates, again, correct me if those are wrong.
BSW is a mess to search, I think I could estimate the number of games played there but I'm not sure it would be worth the hassle.
isotropic has a recorded 12.051.301 games (I believe with all player counts) between 11 oct 2010 and 15 mar 2013. (of which 8.105.394 "eligible" games - I guess this means 2p).
Goko 1.0: I can get a full leaderboard on the Wayback machine dated 17 mar 2015, and it shows 2.044.155 2p games. I really have no idea about when Gokosalvager started keeping track. 17 march is also the very first capture of gokosalvager.com.
An analysis I ran almost two years ago showed that between June 2015 (launch of Dominion 2.0) and April 13th 2016, there were 1.787.686 games. Unfortunately, the Wayback machine can't really help us fill the gap between April 13th and December 31st, because it only saved the partial leaderboard.
As of yesterday, Dominion Scavenger has a recorded 2.301.038 2p games in the leaderboard (29k players). (plays/2)
Does anybody know whether they started recording them on 1.1.17 or later? EDIT: They started on March 31. Thanks JW.
So, to recap:
BSW:
iso: 10/2010-03/2013: 12.051.301
Goko:
-03/2015: 2.044.155 (2p only)
MF: 06/2013-04/2016: 1.787.686 (2p only)
SI: 04/2017-11/2017: 2.301.038 (2p only)
Don't know what to pull out of these data (except that dominion online is less popular today than it used to be), but I couldn't resist the temptation of looking them up.