This seems like a natural evolution of SocialRank, which started out as a side project for identifying your “most valuable follower.” Not surprisingly, it expanded with new data points and filters, though the main buckets are still valuable, engaged, and “best” followers. (The last category being a combination of the other two.) It also developed features to show users the same data about their competitors.
Co-founder Alex Taub compared the new SocialRank Index to “Moneyball for brands” — Moneyball for X is probably a little overused in the startup world, but it’s actually reasonably illustrative here. Just as Moneyball (the book and the movie based on the book) depicts a new take on baseball recruiting that focuses the stats that really matter, SocialRank says it can help brands figure out if they’re really doing well on Twitter, not just by looking at their follower count and the number of engagements on their last tweet, but at more detailed, meaningful numbers that measure how they stack up against the competition, and against the best-known brands in the world.
So the index displays an average for all the brands on a given list — number of followers, number of verified followers, number of engagements, breakdowns by geography, breakdowns by follower count and more. It also shows how those numbers have changed over the past week. This data is provided for free, without registration, but if you want to see how your account compares, you have to log in.
You can read more in this SocialRank blog post.