The Winklevosses already had a substantial part of the project done, and it was more than a dating site. In fact, Zuckerberg copied the Winklevosses feature to connect people. In the absence of competition it seems likely that the Winklevosses product would have been successful. Zuckerberg admits they were well known on campus and would have promoted the site well. The key determining factor in the success of Facebook was not the quality of the software, or even its features, but that it was first social network introduced to the closed Harvard internal market.

http://newslines.org/mark-zuckerberg/?order=ASC