By Blizzard's own words 80% of SC2 players played campaign first and foremost, then coop when it was released.
Only 20% played MP at all, and I'd guess only fraction of them played it a lot.
I.e. SC2 is the game where 80% of sales came from SP players.
I personally know 7-8 people who played and loved SC2 campaign(s).
None of them ever touched MP or ever thought about it. MP-players are a very vocal minority.