Facebook connects people and there are a billion people in India so it only makes sense to get them online or better yet get them online while keeping them in a walled garden for as long as possible so they can reap the maximum rewards out of it.
Facebook portrays this as a humanitarian effort to bring poor people online (see internet.org ads on YouTube which portrays this as giving people free electricity) but it's only for their own benefit in the long run.
It's sad to see that Facebook is trying to kill the same thing which made it what it is. Zuckerberg should wonder what it would be if 20% of India was still using MySpace because they offered it for free in 2009 and didn't give them any option to sign up for Facebook.