The Islamic State of Iraq and Syria (ISIS) has received a tremendous amount of media coverage in the past few years for their successful use of social media to spread their message and to recruit new members. In this work, we leverage access to the full Twitter Firehose to perform a largescale observational study of one year of ISIS social activity. We quantify the size of ISIS presence on Twitter, the potential amount of support it received, and its collective influence over time. We find that ISIS was able to gain a relatively limited portion from the total influence mass on Twitter and that this influence diminished over time. In addition, ISIS showed a tendency towards attracting interactions from other similar pro-ISIS accounts, while inviting only a limited anti-ISIS sentiment. We find that 75% of the interactions ISIS received on Twitter in 2015 actually came from eventually suspended accounts and that only about 8% of the interactions they received were anti-ISIS. In addition, we have created a unique dataset of 17 million ISIS-related tweets posted in 2015 which we make available for research purposes upon request.