I lost my Hong Kong passport with my F1 student visa inside. Do I need to report the case to CBP and ask for a replacement of my I-94 before I can leave the US back to HK? (FYI: I entered the US with an electronic I-94.)
No, your electronic I-94 is valid as long as you maintain your status and stay in the US, even if you lose your passport. If you lose your printed copy of the electronic I-94, you can still use your old passport number to reprint the electronic I-94 as long as you haven't left the US.
It's a good idea to make copies/scans of your US entry stamp in your passport and your electronic I-94 every time you enter the US, and keep these safe so you'll still have them even if your passport gets lost. It's also a good idea to keep copies/scans of your passport. (As discussed on your other question, you will need to replace your passport.)
Edit: The US doesn't have regular exit immigration checks, and you don't generally need to show your I-94 upon exit. But in theory, any time you're in the US you could be asked for proof of having entered legally, for which an I-94 copy is the best proof. In practice the most likely need is if someday you apply for some future US immigration status, as they may request your full history of entry stamps and/or electronic I-94 printouts.