It's not a usual one. There must be context somewhere.

cup, it means who is somebody doing anything? hahahahaha!

nice picture, who let the dogs out :P lol!

There is a popular song in the US called "Who let the dogs out". In fact the phrase is repeated so many times in the song, it is hard to get it out of your head. I guess "who let the jam out" is a play on word after this song.

it's mean who is somebody doing anything