It's an investment. India is an emerging market, and those folks' purchasing power can only increase. It is truly a great strategy, because his company will be the one collecting data on what they like and what they don't before they're even able to buy stuff. That is a sure way to beat your competition.
India's growth rate is a fairly consistent 6-7% and the growth rate of the upper class (read: anyone > 1/2 US poverty level) is probably significantly higher.
Zuckerberg is running a long game here.
Furthermore, anyplace where Facebook isn't huge is a place where a competitor can grow. Note how Whatsapp spread from India/Africa to the rest of the world.
And why exactly do you think Facebook is offering this service? (And, furthermore, why exactly do you think they are offering this service with access only to Facebook and Facebook approved sites, rather than to the Internet at large)?
[On edit: I seem to be getting a lot of downvotes on this post, yet no actual answers to the question]