While the milkshake icon is in part for humor, it doesn't represent the same concept that the other icons represent. The acts of 'sharing' a piece of music and 'sharing' a milkshake are different. When people share a milkshake, the implication is that they're both there drinking it. Sharing a milkshake like you'd share an article would be to give the milkshake to someone else. You wouldn't even need two straws.