It is true that those companies are private entities and are not prevented by the First Amendment (US Constitution) from censoring. However, they can engage in censorship and it can be very harmful to society.
Let's examine what would happen if the leadership of Facebook and Google were conservatives and those companies actively prevented anti-gun, pro-abortion and anti-gay marriage posts from being widely seen (or seen at all). Could that shape the debate within the US?
So, if it would be wrong for them to do that, why is it okay for them to lean the other way and censor pro-gun, anti-abortion, etc. content?
Hint: If you think it is wrong when done to you, you should also think it is wrong when done to somebody else.