|Image via CrunchBase|
The Index is calculated by the number of people who have actively engaged with an employer (comprised of who have researched the company, applied to jobs and who have followed a company on LinkedIn) divided by the number of people who are aware of the company (defined as people who have viewed the employees' profiles and who are connected to the employees) Multiplied by 100 this number gives the Talent Brand Index.
With this index LinkedIn is telling companies that their brand is shaped by employees, and if they actively engage on social media they can push up that numerator.
Recognizing that an Employment Brand is not useful without a context, LinkedIn can offer the TBI based on geography and compared to peer companies as well as by function. If you're a recruiter for an investment bank based in Mumbai you'd want to compare your TBI to other financial companies based in Mumbai and amongst Finance professionals.
LinkedIn has about 17 million Indian members and as more and more join the site, the TBI can become the way a HR leader can track the so far subjective measure of Employment Brand. I am sure that the formula of the TBI would evolve over time as LinkedIn adds more data points - hopefully from other third parties to determine how truly in demand an employer is. It is a great start anyway.
Here's more detail (pdf document) and LinkedIn's site for the TBI