It seems like their big problem is monetization. They're probably reluctant to monetize too aggressively given past experience with companies like Digg, where the users saw the system turning against them and fled.
Somehow Facebook managed to do it, but you still hear grumbling about how big of a mess the news feed is and people searching for something that can replace it.
Well, that was kind of my point I should have expanded on. Facebook, Instagram, Snapchat, et al have a -lot- of interesting tech behind them. From "creepy" suggestion algorithms, calls/video calls, face filters for pics and videos...the list goes on and on. Sure another company could do it, but it's got a higher barrier to entry. Reddit has nothing interesting or proprietary. An unreliable clone would probably take a single person all of a day or two.
Somehow Facebook managed to do it, but you still hear grumbling about how big of a mess the news feed is and people searching for something that can replace it.