The social media blackout that Sri Lanka imposed in the aftermath of the Easter Sunday bombings that killed more than 300 people illustrated the dwindling confidence of governments around the world that Silicon Valley can effectively combat the misuse of social media by terrorists.

Free speech activists are troubled not just by the blackout itself but by the lack of any public outcry in response, said Ivan Sigal, executive director of Global Voices, an advocacy group that favors free speech on digital platforms.