I think the point was to put into perspective the marketshare/growth of whatever platform snapchat is by putting facebook's bar on the same graph using the same scale.

It's like if you showed a photo of several species of elephants to a person who's never seen an elephant, they'd only know that woolly mammoths are larger than African elephants, which are larger than Asian elephants, but for all they know a mammoth could be the size of a large dog. Unless you put something else they already know in there, like a person, they won't know how huge they are.