The rankings are based on overall tournament results, assigning points to players based on placing, player skill, and overall consistency. The system itself serves as a guide for people to see where their favorite player currently stacks up against the competition. It is still a work in progress, and SRK is looking for constructive feedback to help improve the system. You can contact them via their official forums and their contact form.
You can view the complete worldwide rankings for Ultimate Marvel vs. Capcom 3 here. You can also check out the rankings for Super Street Fighter IV Arcade Edition ver. 2012, Killer Instinct, and Skullgirls.
Original Source: Shoryuken