I only received this on two of countless Lyft accounts, so it's most likely highly YMMV, but hopefully some other SDer's got it.
Email Title: Account Update: $100 Added
You've got $100
Take $5 off each of your first 20 rides.
Credit balance as of September 15, 2016. A maximum of $5 will be applied to each ride. Credit expires October 5, 2016 at 11:59 PM. Cannot be transferred or combined with other ride credit. Sample fares are estimates only, actual fares may vary. Subject to Lyft's Terms of Service.