The problem is that Steem is not a traditional equity. It has elements of an equity, but it lacks dividends. It also has elements of a currency, which traditional equities do not.

This makes it harder to compare market cap directly. On the plus side, due to the currency aspect (and the requirement for economic participants to hold for liquidity purposes and normal business operations, which is not so important for Facebook shares), I believe this makes your comparison a gross underestimate.

Another way of putting it: People hold Facebook shares pretty much only to invest in Facebook. In a Steem-dominant economy, there would be far more reasons than that to hold Steem.