Not today, but yesterday, another two U2F keys.
Now Facebook supports two factor authentication using Fido U2F keys, it's slightly more useful than previously, when Google was one of the few mainstream companies that did.
At £8 each, they're not too unreasonable, and with several of them, I can lock one up securely, just in case, and always have a couple of others to carry around (and risk losing).
(I've got printouts of single use numbers, too).Slightly annoyingly but understandably, by design there is actually no way to identify each key uniquely (like a serial number), so I've had to sticker them! I think I should be able to identify each one, by signing a known phrase, which should be unique but produce a repeatable result.
When it's plugged into a USB socket, if a website needs to access it, you get a popup request access, and if you agree the LED on the key will flash. You press the button, and the signed response is set back to the browser, and then the website.