While at NYU Dennis started the Dodgeball, a location based social networking software for mobile devices in 2003. In 2005 Dodgeball was sold to Google. In 2009 Crowley launched Foursquare which was a more expanded version of Dodgeball. In the summer of 2012 Foursquare had 25 million users. He has also worked for Google and MTV. His blog Teendrama even earned him college credit. He lost his girlfriend, job, and home in the September 11, 2011 attacks in New York. He has been named to Crain’s New York Business 40 under 40 in 2011, as well as the same list by Fortune Magazine. AskMen.com ranked him #19 on their list of most influential men in 2010. MIT Technology Review listed him on their top 35 innovators under 35 years old. He was involved in controversy when he produced a fake Boston Marathon bib for his wife in 2014.