LastSeen Updater от LastSeen.me

Once you create an account, install this extension, and login via the extension, it will simply look for activity (activating or creating new tabs) and update your LastSeen time without any interaction on your part.

Absolutely nothing is exposed that you don't allow (and there are few opportunities for you to do that).

It really is an "Am I Alive" to share with people so they'll stop asking.