The best way to cross reference which users are on each social network is to start with your email database. You want to use your email database to help seed your social media accounts.
Identify which social networks your email user base is on by using MailChimp Social Pro (or similar service) and then try and transition them over to that social network. Also, the key is to focus on the top influencers in each social network instead of the bulk using Klout scores.
How to Convert Email Subscribers to Twitter Followers and Facebook Likes (Fans)