It's probably not surprising that it happens. But still, Facebook has promised not combining any Whatsapp userdata and respecting users' privacy when they were taken over[0], and Facebook getting a €110.000.000 fine from the EU for breaking that promise three years ago [1].

[0] https://blog.whatsapp.com/setting-the-record-straight [1] https://ec.europa.eu/commission/presscorner/detail/en/IP_17_...

Well, that doesn’t sound like a well thought out software architecture does it. What did they do to get 2 completely separate applications mashed up together so badly that it “costs billions” to untangle?

That sounds like Facebook's problem. "There is no 'it's too hard' defense", as Tim Wu notes.