I haven't played Fortnite, but from what I know it took a somewhat resurgant form of gameplay (battle Royale) and added some innovations to great success.

I can see Fortnite having a multi year run, but gaming is a completely different medium to a traditional 'social network'. Maybe this says something critical about social networks, mainly the fact that a successful social network must be oriented around passive consumption rather that active participation.

I'm sure folks watch Fortnite videos online and live streams, but I get the sense that a large number of folks have spent more time playing that watching. You cant really say that for most other social networks, especially when you remove 'power' users such as influencers and brand accounts.