There's no "free speech" on television; it's censored all the time (as a matter of course, even.) There's no "free speech" in newspapers; there's no "free speech" on the radio.
These are all commonly censored due to government regulation or when the company running the media outlet determines that broadcasting speech could be detrimental to its own interests.
How, exactly, are Facebook and Twitter any different from any other media outlet that solicits public content, then publishes what it wants? I think you're confusing what it essentially a self-publishing service with a soapbox on a streetcorner. With the latter, you generally do have free speech (within limits), with the former you certainly do not.