Facebook says breaking off WhatsApp and Instagram would cost billions of dollars (opens in new tab)

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.