I'd wager that that'd only hold if all (or a lot) of your Snapchat contacts did the same. I think people often overestimate people's unwillingness to use multiple apps while underestimating their unwillingness to switch to a different version of the same app.
See that's a seperate app that exists on one platform, while FB Messenger is cross platform and already used by most people, so adding one feature (while anathema to the way companies are doing this atm, opting to make each feature an app) would instantly overtake a lot of the value of snapchat.
