Well "locking in" is just a phrase, I think that motivating them to stay on is the same thing. Facebook shows you only the content you actually enjoy and if you get hooked on watching viral videos and drunk people falling on tables then that's what they show and they retain users hard with it. Ultimately leading to more likes, comments and engagement.
But I agree an invitation style is going to be the way
The hardest thing in the world for people with a technocratic bent – and let's be fair, most of the people involved here definitely lean toward technocracy – is to accept that other people like things they don't. There's a lot of assumptive judgment about what things "should" be valued and what things "should" be floating to the top, and what things people "should" be doing here.
Ultimately that is going to be a pretty toxic mindset to developing a platform and a system which people want to engage with their way. And "having it your way" has a long and storied history of being what people want.
In that context I am generally extremely careful about what phrases I use to think about and approach problems where I know I have a non-helpful prejudgment.
Would I prefer that people didn't preferentially vote up pictures of cats? Absolutely. They do nothing for me. They bring me no value, and as such I don't think they should be rewarded. But does that translate automatically into believing that no one else should want to vote up pictures of cats? Or links to news articles that they find compelling? Or a cool meme that they found? Or a long, rambling article on web of trust? Or a serialized novel, one page at a time? Or…?
You see the problem.
So let's avoid "locking in" and pivot to "giving people a reason to stay." If nothing else it will keep us honest with ourselves.
Well worded, respect. I hope the idea can develop for sure