[ratbr]

I think FB does not have to be a PayPal, and there are many good reasons outlined in these comments already why that is a bad idea.

However, it can become some kind of market place. It already knows user locality, it already has people flocking to it. It already has big brand stores sharing discounts and coupons to consumers.

All it needs to do is finish the last mile and allow those brands to actually have a "Facebook-only Sale", for example.

Also, it can become a great "local market place", and can help local businesses to sell to local customers.

TLDR: not just ads, and not necessarily a payment gateway, but a niche market place could be it.