For things like Facebook it's sensible to keep file sized under 100kb -- and quality can be quite acceptable in 25k. You can get the camera to reduce quality or you can resize shots in Windows Paint and also alter the JPEG file size by quality in Photoshop.
3M of 640x480 mean that the camera's maximum quality is 3 Mega bytes at 640x480 pixels depending on the image taken, more colors means more file size of the image. An identical photo of the same image size, higher file size (kb) means more vibrant and less hard edges. A high quality 640X480 is around 300 Kb or more, Medium quality is around 65 Kb and low quality usually max at 50 Kb.