score:6
You are supposed to turn it in to the airline, but it is no problem if you didn't. The US Government states (https://help.cbp.gov/app/answers/detail/a_id/752/kw/I-94%20was%20not%20collected, second paragraph):
[...] If you departed by a commercial air or sea carrier (airlines or cruise ships), your departure from the U.S. can be independently verified, and it is not necessary to take any further action, although holding on to your outbound (from the U.S.) boarding pass - if you still have it - can help facilitate your reentry next time you come back to the United States. [...]
I recommend to staple the outbound boarding pass right onto it (and keep it), so you have it if ever needed; otherwise you should be good.