Facebook's interest is in capturing more market share and maximizing their profits...not sure how that aligns with the interest of society, but I guess it sounds good to call it promotion of free speech.
Social media is a better platform for political ideas than newspapers or television. Every newspaper and TV channel ends up slanted towards one side of the other. The NYT goes for the Democrats, Fox News goes for the Republicans.
But social media is whatever you make of it. You can get either Democratic or Republican viewpoints from Facebook or Twitter.
I don't know why people keep saying this, because it's obviously not an accurate description of how people are using the term. Freedom of speech is a principle that certain people believe in and that the US government implements in the form of the 1st amendment.
Problem one is less controversial: small groups of propagandists game the algorithms to gain more exposure than they could get organically.
Problem two is less comfortable: sometimes content that appeals to the majority does so at the expense of a minority. An algorithm that is purely democratic is only as fair as the majority is altruistic.
What is often missed in the Free Speech discussion is the concept of the "marketplace of ideas", that is the importance of Free Speech is allowing the market to decide which ideas/speech is the best. The problem with FB and all other platforms is they are replacing the market with their own algorithms to maximize advertising revenue.
Facebook standing for Free Expression is such a play on words...Facebook stands for monetizing the content users give them for free.