If Google, Facebook, and Twitter are editorializing content, such as silencing right wing views and anti-immigration sentiments (Zuckerberg met with Merkel to discuss implementation of this, for instance). Then theses companies lose their DMCA Safe Harbor Provisions which safeguard a platform provider from being liable for user generated content. Under the DMCA such media platforms are not responsible for the content posted by their users so long as they are not editorializing the content, such as filtering posts along political lines to present a political bias.
If DMCA Safe Harbor provisions are stripped from Google, Facebook and/or Twitter then THEY ARE RESPONSIBLE FOR SPREADING ISLAMIC TERRORIST PROPAGANDA AND INCITING VIOLENCE.
It's not a DMCA provision. That's a safe harbor for copyrighted material. What you really want is the Section 230 safe harbor, which states that "No provider or user of an interactive computer service shall be treated as the publisher or speaker of any information provided by another information content provider.". A social networking service clearly falls under this provision.
In space, no one can hear you fart.