What is the use of saw-teeth present on the nacelle outlets of modern turbofans?
These "saw-teeth" or "chevrons" or even "acoustic liners", are there to help reduce noise generated from the operation of the engine. 

As hot air from the inner core of the engine mixes with the other, cooler air, it generates noise. These "saw-teeth" are there to help smooth that mixing, thereby generating less noise which is the result of turbulence. These are seen on the Boeing 747-8 (as seen in your picture) and the Boeing 787.

The reduction of sound generated means that less noise insulation is needed for use in the fuselage of the airplane. Less insulation means less weight. Less-weight equals less fuel burn, which means that chevrons are great for passengers, and economics.

There's only one difference from your picture. As much as we would like them to blow kisses to everyone, that's not what they are there for. :)