That is easier to do when your community covers some particular niche. That effectively does the ad targeting for you. An auto parts store knows that most people in a forum about car repair might actually be interested in their product.

It's harder with a more general purpose community like Facebook or Twitter. Most people on these platforms probably have no interest in auto parts. A good chunk of them might not even own cars at all. Initially, this meant that impressions sold in these spaces were dirt cheap, because they so rarely converted into clicks. This is where data collection comes in, because it allows your advertisers to actually narrow the focus of their ad campaigns to users that might actually be relevant. And the more data Facebook has on you, the more detailed and effective these campaigns can become.