Determining usage and growth of Free software has always been a challenge. For over a decade, arguments have been held – sometimes flamewars – whose central point was the usage level of software that is freely distributed.
While market share can be estimated based on sales numbers, Free software usually replaces existing software that is proprietary, i.e. its ownership lies with a vendor and it is usually treated as an integral part of another product.
When discussing Free software, the term "installed base" seems rather popular. It is installation, not embedment or preinstallation, that tailors a product to the owner's personal needs. Unfortunately, installed base, as opposed to market share, proves to be a tricky thing to gauge.
At the center of this debate, one typically finds the GNU/Linux operating system.
More Here .