> The reason a LinkedIn competitor hasn't emerged is the same reason a viable Facebook competitor hasn't emerged: at this point the network effects are too strong, and it's extremely difficult to beat incumbents because of that.

I'd say LinkedIn actually has stronger network effects than Facebook, or other consumer social networks. Consumer social networks are used more casually, so it's easier to get users to sign up than it would be for a professional social network, where users tend to adopt a more conservative mindset. And we've seen several consumer social networks rise in the post-Facebook era - Snapchat, WhatsApp, and Instagram.