From the PIC18F2455 data sheet (AFAIK the FB uses the PIC18F2455), they recommend a max series resistance of 2.5kR (R=Ohms), and....
"A maximum source impedance of 10 kΩ is recommended for the analog sources"

The only other parameter I can find is an internal ADC input leakage of 100nA (and 500nA for the comparators). 100nA is equivalent to 50M-Ohm leakage.