According to Twitter, they have deleted over 125,000 accounts since mid-2015, saying those accounts are linked to terrorism, and that terrorism violates their terms of service. The company reported increased hiring on their anti-terror teams.
ISIS is known to use Twitter for spreading reports of its activities, and in recruitment, and some of those accounts are pretty obvious and in many cases fairly well-known, with Twitter in the past leaving a few such accounts alone for the sake of US intelligence agencies tracking them.
But a lot of these accounts apparently aren’t so clearly ISIS-linked, with Twitter conceding that in many cases it is “a judgement call” whether a tweet is simply controversial, or amounts to “extremist behavior” worth banning someone over.
The company concedes there is “no magical algorithm” for spotting terrorists, but is willing to shut down certain accounts to meet the “needs of law enforcement.” The Obama Administration has recently been pressing top Internet companies to “make it harder for terrorists to use the Internet.”