The problem is no different from the issue we see in mainstream media. When our information flow is dominated by for-profit institutions, the information is biased towards profitable agendas. See: Tesla coverage bias, Cambridge Analytica, political debates favoring incendiary soundbytes because intellectual discourse isn't profitable, etc.
In Mark's shoes, I'd publicly acknowledge the importance of democratic information flow, ideological equality, etc and implement restrictions on the reach of any individual organization (or even ideology). I don't care if you have $1B to spend; you can only ever push your agenda to 1% of our audience at the same time. It's the ideological equivalent of an income cap.
But of course, Facebook, Inc. is incentivized to reframe the issue of "avoiding responsibility" in terms of "avoiding censorship" because avoiding responsibility is more profitable. Why not compare what they're doing to an oppressive dictatorship to really drive the angelic behavior home, while we're at it.
How is it possible to uphold freedom of speech when you allow someone to purchase my effective silence?
Obviously this would be challenging ~ how do you categorize ideology? ~ but I think at least starting to think about ideological equality in digital spaces is a worthy target of our cognitive and economic investment.