Most Uber discounts are only for brand new sign ups.
The Flight Deal is featuring a promo that applies to existing Uber riders. You can get a free ride of up to $15.
The instructions are a little complicated, but the Flight Deal was able to get this discount to apply to over a dozen phones. Only one attempt failed (so YMMV because you may have something in common with that one phone).
The deal requires both an Uber app and the latest version of Google Maps.
According to the Flight Deal, it looks like the promo code is good until the end of the month.