They seem like they’ll spread to everyone and stick around forever.
This almost never happens.
In order to spread to everyone, they need to move beyond the people who are looking for a new idea. And that happens when existing users have a powerful reason to tell their friends.
Not only that, but the idea has to solve a real problem for people who weren’t sure that there even was a solution to that problem.
And in order to stick around forever, there needs to be a generous lock-in, a reason to not only keep using it, but to not switch to the next thing.
The network effect plus stickiness almost never happens after the idea is launched. It’s about marketing (in the powerful, design sense) not promotion.