I don't know what others experience is with their friends, but aside from RTS games, Multiplayer FPS games on the PC are just about the last place my casual friends last, I'm certain more of them play a MOBA than an FPS. My thought was, what percentage of Blizzard's fanbase (Diablo/WoW players) are going to regularly play Overwatch? I think less than 20% of my Bnet friend list would play Overwatch weekly. I think Overwatch will be successful in its own right, but I'm not sure its playerbase will overlap with some of Blizzard's most rabid fans. A massive percentage of WoW players can't hack it in hardmode raiding, will they persist in an KB/M FPS environment?
That's honestly my biggest question for the game as far as projecting numbers. TF2 tops out currently at like 50-70k concurrent at most. CSGO is by far the biggest FPS for concurrents but it is in another stratosphere of difficulty for the average Blizzard fan.