How much memory can be saved on making all the icon pics required on putting in a png format?

Question

Sites like facebook,google plus have there whole images in one picture how many bytes of memory can be saved using this method

Answer 1

You are not saving any memory. You are saving bandwidth because you are making fewer HTTP requests to the server.

As pointed out by @iccthedral in the comments section you are actually saving bytes because you now have a single image => the total amount of saved bytes = numberOfPictures * sizeof(pngHeader).