If you click "Profile", then "Forum Profile" in "Modify Profile", there are a list of things you could fill in. For example, "AetherCards Products I Own", "Facebook", "Found The Discourse via", "My Content", etc.
Some of these would show up below your avatar in your posts, some would show up the same place as your signature, and some would show up in your profile page. The "Google", "YouTube", "UC" you see would fall under "Found The Discourse via" section and show up at the signature.