Uptime might be somewhat ephemeral, but it can be a pretty clear signal to indicate that two different user profiles are actually the same person. For example, it would be a plausible explanation for why even when I took several steps to hide my identity when creating a new Meta account (new Safari profile, tracking blockers, VPN, anonymous email account, etc), Facebook immediately suggested all of the contacts from my other account as friends. Or perhaps they picked up on some other signal. My point is, I absolutely don't consent to my devices sending their uptime to third parties without explicitly asking me every time.