It was very satisfying to see algorithms converted into practice, touch millions of users, and help monetize a service that has become a societal force. Twitter was small, but the speed at which it executed in converting algorithmic prototypes into products surprised me. Part of this was the amazing executives, PMs, and engineers I was working with at the time (Dick Costolo, Anamitra Banerjee, and Utkarsh Srivastava on ads; Ev Williams, Josh Elman, Pankaj Gupta, Abdur Chowdhury, and John Sirois on recommendations). But a part was definitlely timing. I had the option to go to Facebook for a similar position. Facebook had by far the better infrastructure and the bigger engineering team in 2009, but they were well down the path of both recommendations and monetization.
The experience also shaped my research in three ways. First, it gave me research problems to work on. Three papers I am immensely proud of (Incremental PageRank with Abdur and Bahman in 2011; Precision of social networks with Aneesh, Kamesh, and Reza in 2013; and Fast Personalized PageRank with Peter, Seshadhri, and Sid, in 2014) came about because I had a better appreciation of the right research questions to ask, from both a theory and a practical angle. Second, it gave me confidence to work on big problems, leading to me jumping into crowdsourced democracy. Third, it helped me realize how important team work is -- I expect more of my students and collaborators now and hold them accountable, but also place more trust in their ability and judgement.