[eva1984]

Instagram is pretty trivial: ads. AFAIK, their user base is largely female, and this catalog of inventory is very much desired by advertisers.

In terms of Whatsapp, micro-transaction maybe? That I am not quite sure, it is not going to be as straight-forward as Instagram.

[forgetsusername]

At some point the "throw ads in it" model is going to stop working.

[joosters]

Whatsapp used to be monetised before Facebook bought them, so in theory it shouldn't be that difficult to go back...