Flexjet and Netjets are fractionals...
so in order to use them, you have to BUY a fraction, THEN pay a monthly management fee, THEN pay a per-hour usage fee... That is how they make money!
There is no such thing as a "profitable" charter... you have to charge so much in order to make a profit that someone would just go to a 135 company that is just offsetting costs for some owner-- just like slyguy said.
But that 135 company is charging a hefty monthly mgmt fee, plus taking 15-20% of revenue made from charter, so the mgmt companies are making out like bandits while the pilots do all the work!