Good point. There's a precedent for separating industries when it's in the public interest [0]. I think the argument needs to be that housing both industries under a single roof creates a conflict of interest that's too strong to be managed through normal regulatory actions like fines.
There's also a precedent for preventing companies with a monopoly in something from expanding into other businesses, like Bell. Clearly facebook isn't a recognised regulated monopoly like that, but there are similarities.
[0] https://en.wikipedia.org/wiki/Separation_of_investment_and_r...