Reliance anyway would have had to pay for marketing, and Facebook provides them that.

So they divert their marketing budget to provide some free data to users as free basics.

It's effectively bartering marketing and data costs. Stakes for Facebook in it are high, so I wouldn't be surprised if they even paid for all of the data of the users.

If FreeBasics get through then reliance could package others like Youtube package, netflix package, full speed package etc.

Attracting non-data users, and hoping they will start paying if they like it.