1. Popularity follows an extreme power law, so the number of people that would need to move is smaller than you'd intuitively think
2. Fads change relatively quickly, so it's not even necessarily about getting people to move, but about getting the next generation to invest somewhere else
3. The big players are extremely metrics driven, so if people feel like the tide is shifting, the transition from trickle to deluge could be shockingly fast
It's not all theoretical - Instagram created an entirely new network effect very quickly, and only FB's ability+willingness to gobble it up kept it from being another major player. But the next thing will come soon enough (less than 10 years for sure, probably less than 5), and then the dynamics of the space depend on the whims of a South American butterfly (or SF Biz Dev)
Networks effects.
More precisely, The fact that you can't read and reply to someone's tweets on a clone, due to Twitter's terms and conditions, and copyright law (enforced by states).