'sponsored likes' on facebook?

[juggler of montreal]

when one of your friends likes a product on facebook and it keeps appearing down the side of your homepage like an advertisement... eg 'Sadie McKenzie likes Heinz tomato ketchup'.... what's that all about? Does it happen automatically whenever anyone 'likes' a commercial product, or do facebook users do it on purpose? At one point I wondered if people were getting paid to 'like' things, but then I thought that's fairly unlikely...