* 11 pages, 13 figures. Published in the Proceedings of the 12th
International AAAI Conference on Web and Social Media (ICWSM 2018). June
2018. Stanford, CA, USA Access Paper or Ask Questions
* A preliminary version of this paper appears in the Proceedings of the
24th Network and Distributed System Security Symposium (NDSS 2017)
[arXiv:1612.04433]. This is the extended version Access Paper or Ask Questions