Facebook makes $7 per month per user in the US and Canada. So, it's likely fairly close though.
I think YouTube has something like this (in the US) where you can pay monthly for an ad-free experience?
There must be techniques to get around that. Maybe grant free or discounted accounts if you're connected with enough subscribers? Allow subscribers to gift free memberships to their friends? Maybe have a long "initial trial" (e.g. totally free for 1 year)?
I would make sense to me to scale the price to the market. Make it $5 in the US and 5 rupees in India.
I too would gladly pay (anything reasonable) if it meant they'd split off from Facebook and become in dependent again.
The reason whatsapp got traction in the first place was it was free (for a year+) when SMS was not. If Whatsapp cost $1 then people would use whatever was free.
BlackBerry was the combo of the handset + the backend services hosted by RIM including BBM and people did pay a monthly fee for access to the latter (over and above regular carrier charges)