What prevents the separate entities from forging close partnerships?
I don't see how breaking up FB would work in practice. Maybe someone more knowledgeable could clue me in.
Nothing would stop close partnerships but GDPR-style regulation would hopefully stop the most egregiously invasive kinds of data brokerage.
The truth is that regulation is always and everywhere a game of whack a mole, and as long as capitalism exists someone will have to wave the comedy mallet around.
Normally when businesses are split up, they are broken into multiple pieces that each have their own revenue and are viable standalone businesses.
In this case, the ad-selling part of Facebook is the entire business as far as financials are concerned. It's the only material source of revenue for Facebook. If you were to split the ad-selling wing from Facebook, then the "remaining part" would have no revenue. A regulator action along these lines is tantamount to crippling or shutting down the business.
Source: Facebook Q3 2018 earnings show that the only material revenue is advertising at $13.5 billion (Q3 2018): https://investor.fb.com/investor-events/event-details/2018/F...
For comparison, Microsoft reports earnings from many separate lines of business including Windows OEM, Office, Office 365, Enterprise Services, LinkedIn, Azure, server products, Gaming, etc. https://www.microsoft.com/en-us/Investor/earnings/FY-2018-Q3...