If a messenger could overcome FB, snapchat might actually have a chance, but the key value proposition that FB offers is that it's your online persona. You have pictures up from all the years you been around, you have your friends' profiles to go check out at will, you can start a group, or make an event and know all your friends are already on Facebook to see it.
So why buy? Messengers have the highest retention rate. You can see them expanding upon it by integrating FB profile + allowing you to install "apps" or "plugins" that do X/Y/Z. There is a Chinese company, WeChat, that has made billions on this model. I'm sure FB would like to as well, but with it integrated with FB login etc..
Downvoters: Genuinely curious how I'm wrong if you feel like throwing in your 2 cents.