There are two problems with this argument. First, there is nothing unlibertarian about criticizing private businesses or using peaceful and voluntary means, such as boycotts, to persuade businesses to change their practices.
The second and most significant reason the "they are private companies" argument does not hold water is the tech companies' censorship has often been done at the "request" of government officials. The extent of government involvement with online censorship was revealed in emails between government and employees of various tech companies. In these emails the government officials addressed employees of these "private companies" as though these employees were the government officials' subordinates.