Personally I'm not a fan of social networks and never use them, but there are 2 reasons why I want you to login with existing social network account:
1. Strictly technical - I don't want to implement password recovery and mail verificaion because of a lot of problems with spam filters and spam blacklists that are so popular nowadays. It's just too time consuming to get server IPs out of public blacklists because of some idiot who reported perfectly good e-mail as spam.
2. More important reason - this site is intended to gather sometimes sensitive information about businesses and products and I don't want it to be spammed in favour of or, vice-versa, used for unhonest defamation or some other kind of black PR. With social networks accounts you can at least see who is providing informaion in doubtful cases and tell who's real and who's fake.