Why do you think steem might win over Facebook or anyother social network app that currently available on the open web? What would drive the average consumer to use a PAID service if there is a free service that already exist.
Never heard about Steem, so I went google it, after I saw this post. Although the project looks interesting I think it will fail because of the community it will attract. On facebook, users share posts and photos etc because they want to, and because of the status it offers them. Some like to connect, online, and most will probably just want to brag about their achievings and get more popular. I don't say this in a bad way, I guess it's just something humans have (some more than others), but we all like attention.
As for Steem, people won't really go for those things. They will get probably more attracted by the possibility of making some money, and will probably not even care about anyone's posts etc, so I guess it will never work as Facebook does. It might be ok for advertising etc, if it ever gets a big audience, but I doubt it will ever be an alternative to Facebook.