What do the thumbnails/icons of your friends on an artist's page represent?

[aranknowles]

On a computer, when you're on an artist's page, to the right of the page there sometimes may be little circular images of your friends. I was just wondering what do they mean? I'm pretty sure they don't mean that your friend follows this artist; and they can't mean that your friend has listened to that artist's music because I should see them more often. So I was just interested to know why they're there and what they mean?