A solid point, and one I've often thought about.

I guess by that point steemit (or whatever site that runs on steem takes its place) will have to be as good as facebook, so that people will switch for other reasons: usability, aesthetics, etc.

Also, don't discount the fact that you're assuming all those people will register investment-less. But if steem becomes viable as a currency used in everyday life, you'd hold your "facebook" money in the same wallet you use to pay for your Starbucks, so why not use the same wallet to vote, or outright tip people? People pay for angry birds lives and stuff lol! They pay for digital gifts! People could load up their accounts to get the pleasure of giving people a few cents with every vote. It might become an addictive game-like experience for some.

So steem would be an aspect of their everyday financial life, and using it on a social app would just be icing on the cake.