When a friend shared a Facebook post with Michelle Burris inviting her to protest in downtown Washington, D.C., last Saturday, she knew she had to go. So she bought a Black Lives Matter mask from a street vendor before marching the streets of the district with a "No Justice, No Peace" sign.

After that march ended, she pulled up details on Instagram for a car caravan demonstration just a few blocks away. "It was extremely powerful, not only Facebook but Instagram," Burris said. "It was very easy to mobilize."

Protesters are using a variety of technology tools to organize rallies, record police...