Free speech by definition affords citizens the right to speak their mind. However, it does not mean they can do so without consequences.
If someone wants to express themselves online, they are free to do so without the threat of government censorship. However, if those opinions are inflammatory and spark social outrage, the users that are outraged by the statement are entitled to express that outrage.
Self-censorship out of fear of generating a negative public response is not an erosion of free speech. It is the public sphere setting social boundaries on what is acceptable and unacceptable. For it to be an erosion of free speech, the state or another entity would have to restrict the expression of certain beliefs. Because people are free to express whatever opinions they choose online, including anger, free speech remains intact, even in a climate when publicly abhorrent views are challenged through social media outrage.