WeChat solved a problem specific to the Chinese market, given how it's internet market is structured. In the rest of the world, that problem is solved at an OS or browser level. And buying either of those would have been way cheaper than $44 billion
The thing is with both OS and browser is that they are really long term efforts and I am not sure if Elon could pull that of anymore. For sure Elon from 10 years ago could, but not sure if he has the patience anymore. Same with Zuck, I don't think he has the patience for what he is trying to achieve.