score:7
Almost certainly your "exit" was recorded when you left the country, despite the fact that they didn't take the card. The airline provides these details to the US government, and they are tracked using that rather than the I-94 itself (this is one of the reasons that the I-94 cards are no longer used when entering by air).
Your can confirm this by going to the CBP I-94 website and clicking on "get their I-94 Number". You can then request a "Travel History" which will show all entries and exits from the US.
If your exit is correctly recorded then you can simply throw away from I-94.
If the exit has not been correctly recorded, but you have subsequent entries/exits (which it sounds like you do), then you can also safely throw away the I-94 - but I would suggest keeping a copy of any "proof" that you have related to when you left the US on that trip (eg, copies of flight itineraries, tickets, etc) just in case it ever becomes an issue - although the odds of that are extremely low.
As a more generic answer, if you have not had any subsequent entries to the US then you can post the I-94 back to the US DHS/CBP, as described here.
Upvote:2
I always leave any stapled stubs and whatnot in my passport, and don't mess with them. In your case, the next time you enter the US the immigration officer will probably take the remaining I-94 (and perhaps ask you what happened when you actually left from that visit). If they don't take it then they probably don't care, since they've migrated to the electronic system.
Upvote:4
According to the current version of this CBP FAQ:
If the traveler departs through an airport or sea, they will not need to turn in their form, as Customs and Border Protection (CBP) will receive the information electronically from the airlines.
However, this information may only apply to departures now (mid-2014), and not earlier like in August 2013. I remember reading from somewhere earlier that you should turn it in if you got a paper I-94 when you entered, but not if you entered with electronic I-94.
In any case, it probably doesn't hurt to mail it to the address indicated anyway, with proof of travel, and also to permanently keep airplane boarding passes from those trips in the future for when you apply for U.S. visas and enter the U.S.