I think this is the key.

The hook that you describe above is a sense of community - their friends are here.

I was taking night classes when Facebook first rolled out so I got an invitation to join in their earliest days. I think the reason for their virality was that they did it one university at a time, so groups of people that already knew each other in real life and had common interests were all joining at the same time. They were both exclusive (Only "your school" is invited) and inviting.

I don't know how we accomplish it on Steem, but I think it has to be about reinforcing real-world connections and building a sense of community if it's going to be lasting.

Edited to add: This line of reasoning relates to my recent thinking about voting strategies to support authors who are network builders.