Obviously due to the open nature of Linux, the hardware vendors can't go to one distributor and ask about their total sales/downloads. One of the common methods that I know several IHVs use to gauge the market-share of different operating systems -- at least in part -- is by looking at web-browser statistics. Not of the breakdown of traffic from Linux, Windows, Mac OS X, BSD, etc to their own web-site necessarily, but to mainstream web-sites. Sites that are very common to a diverse selection of users and not a particular vertical market. Of course, this isn't the most accurate way of measuring for the PCs not connected to the Internet or for those that do most of their web-browsing from a mobile device or at work, but it's one of the leading methods.
Linux users commonly tend to feel e-mailing the company directly will also improve matters if you're in the "alternative OS" crowd, but in most cases it doesn't end up working out. Unless there's a serious undertaking in mass emailing of these companies, nearly every time the effort is wasted unless the company replies and drops a tad bit of interesting information (and you should always pass that along to Phoronix ;)). Petition lists are also common for Linux users, such as OpenTheBlob.com for NVIDIA, but those tend to have little effect as well.
As I've commented before, many consumers also approach the wrong company when lobbying for greater Linux / open-source support. In particular, if you're working towards better Linux graphics drivers, contacting AMD or NVIDIA directly usually will have little impact. The more appropriate solution is contacting the AIB (Add-In-Board) partner of AMD or NVIDIA from who you purchased the graphics card. That's AMD/NVIDIA's real customer as you're not buying the graphics processor directly from them. If a company like Sapphire, ASUS, PowerColor, or eVGA goes up to the IHV and demands greater Linux hardware support, that's what facilitates real change. Likewise, if you bought a PC from Dell, HP, or ASUS, your feedback communicated to the IHVs from them is what carries weight.
Some threads worth reading that are along the same sort of lines are ATI dropping support for Linux?!, Linux Desktop Marketshare Passes 1%, Future of my support for ATI, and donations for OSS drivers. From the Phoronix Forums you can also get comments from the developers and hardware vendors themselves.