[evgen]

Have left and have been working on my own startup for several months now, but I am aware of how some of the data is siloed at FB and also that WhatsApp was run very independently (as in, almost no operational contact between the two companies) up until the summer at least. I don't doubt that FB can tie a specific FB user id to a WA user id, and if they tried hard they could tie a large fraction together using a variety of data sources (certainly not a 'one-line SQL query') but between 10-25% would fail due to bad data on either the FB or WA end. A partially complete data set is not as useful as you might think...