I did work for a few months in Bombay last year, and most of my colleagues (20s-to-40s middle class white-collar employees -- a minority in India) had a FB account, but with very little engagement. I have to assume that having many non-urbanized people in India, and therefore many less-connected relatives, probably reduces the interest in FB. They were however all heavy users of Whatsapp, which makes sense to me.

I have a feeling that, thanks to smartphone adoption, legions of people are counted as 'in' by FB only because the app is running in the background, when in fact it's not seeing any significant use.