The 5V pin can power a small number of devices, or low power.
The 5V pin should not be used to power the Mega2560 itself - if you feed in 5volts here then you are also "back-feeding" the onboard 5volt regulator which will be wasting some power and bypassing onboard protections.
The best way to power the Mega2560 is with 7V to 12V in the barrel jack. I use about 8 or 9 volts. There is also a Vin pin which works but this bypasses a protection diode.