I have no idea how many accounts have been compromised but I don't think it's as bad as you're claiming.
I've noticed a surge of dormant accounts joining in Bitmixer, all of them shitposting and most of them having the same/similar posting patterns (e.g. inactive since X month, start posting after Y date). I think the OP is at least somewhat correct with his statement. Some of those dormants accounts could have been reactivated by real owners who happen to have heard the recent news and getting interested in bitcoin again.
Doubtful that account farmers activate only when there is Bitcoin news around. 
I mean the bitcoin price is enough to motivate them to posts again.