Well, when comparing PS3 sales to PS2 sales there is one thing we always have to remember:
What made the PS2 so succesfull were not it´s huge weekly sales, it was the longelivity of the console. This is what the Playstation 3 is gonna lack in the end, probably. It has much more competition and isn´t the leading console. Therefore it probably won´t get that much support after year 3 or 4.