Facebook is valued so much because it steals people's information and sells it to advertisers for extremely targeted advertising. Steem doesn't do that. It's not just about the amount of users, it's also about how you monetize your product. Right now the only way steem is monetized is through the "promote" tool, which tbh isn't that attractive. While the community has offered many different options to attract advertisers investments, I haven't seen the dev's mention this problem at all.