1. Content Quality reduces due to lack of choice, and lack of exposure: Closed-groups are places where spam, false information, propaganda breed to a high degree. Other parts of internet help verify claims/suspicions/hypothesis.

2. Sets up a monopoly, vendor-lock-in, changes habits: Facebook would hold immeasurable power, once they've got people hooked onto thinking that there's nothing beyond their apps. The content they censor, never appears on their networks; the content they don't like die a bad death, the apps they don't interface with die a rather quick death, the business they don't bless die a death by the guillotine, the businesses they do business with die a death by a thousand paper cuts.

3. MBAs ride the wave with their business plans, corrupt official enact new laws, freedom is lost: Similar schemes by multiple powerful corporations crop-up everywhere. Internet gets fragment, and everyone is left to rue the freedom they gave up. 'Cause its a long battle up the hill, once the rules and laws are in place, they're very difficult to change because of the inherent momentum it affords to all those "rent-seekers" and "opportunistic" businesses that crop up around it. For a case study, refer how the Airline industry squeezed out every ounce of leg-room, and comfort, for more seats, and in hope to turn more profits...

There's a lot of other things that might happen, or not happen at all... but, remember, when something's free, the product is usually the user.