For me it is not 100% clear how Datum will collect the data en store it, without FB/Google taking it too.
Now that you say this, I became very curious how they wil collect data before FB/Google takes it.. I read the whitepaper again, but all it explains is how datum encrypts data etc.
Only thing I can imagine is that Datum saves info you post and sells it (if you want) to other parties for cheaper than facebook or google price.
you can block Facebook and google with scripts how a adblocker do it.
so maybe DATUM will block first the transmission of your data.