As it was explained to me, he finds the risk to his main desktop environment of browser fingerprinting/tracking by fb and its tracking-pixel using "partners" to be unacceptably high. By only using the browser in the vm to login to facebook and never anything else, this diminishes the risk.

Never heard of uBlock I suppose, poor lad.

uBlock is not bulletproof as it relies on signatures. Not to mention, the absence of data is data by itself, so maybe it's actually better to provide data, just that said data is fake and dissociated from your primary identity.