Zelle (possibly just "as implemented by the banks I've interacted with") has very small limits - a couple thousand dollars.

Are people really going to make big transactions using Facebook money?

People will make whatever size transactions they need to make by whatever mechanism they trust.

If Zelle cannot make the transaction happen, it is out of the running. Full stop.

> with free instant transfers

I've noticed a "fee: zero" statement on the transfer page recently. I expect there will be an actual fee in the near future.

U.S. credit cards extort big fees from businesses and give kickbacks to consumers. Hard for "free" to beat that, if consumers have the choice.

It may change now that businesses are allowed to pass on fees.

Zelle is only for P2P payments, and was not initially available at every bank. Additionally, some banks market it differently. Not everyone knows the service they are using is Zelle.