Terrorists are still successfully using the internet to communicate with and recruit followers, despite progress by big tech companies in cracking down on the activity in recent years.

Online terrorist propaganda is attracting fresh criticism in the wake of Saturday’s deadly terror attack in London. Prime Minister Theresa May and others have singled out Silicon Valley for criticism, saying Facebook Inc., Twitter Inc., Alphabet Inc.’s Google and others need to do a better job policing content. It is unclear if the U.K. government...