In the US, I would guess, it's a smallish percentage of VFR flights, but all IFR flights. The reason being that VFR advisories (flight following) is generally available and provides a similar level of protection with less formality.
In Europe it seems to be seldom used except where legally required e.g. for flights crossing an international FIR boundary and IFR using controlled airspace.
There is less radar service available to VFR flights in northern Europe but also much less wilderness from a SAR point of view.