One of the most common arguments for using Facebook is a billion people use it.
A billion people also watch TV, use mobile phones, walk down the street, eat bread too, but we don't advise organisations to spend more on billboards, adverts, sms ads etc…That's because we can't reach most of them.
It doesn't matter how many people use the platform, it matters how of them you can reach.
People aren't browsing Facebook looking to become fans of pages. So if you want them to become a fan you usually have to send them from somewhere else.
And if you're sending them from somewhere else you might as well send them to somewhere else.
That somewhere should be a place where you have control, where you have greater than 1% reach, where you can shape and manipulate the environment to suit your needs.
There may be good reasons to use platforms like Facebook (groups), but the number of people using it isn't one of them.