Tablets aren't the only devices taking sales from PC makers. Google's (NASDAQ:GOOGL) Chromebook is also taking a toll, if comments from Acer are to be believed.

Chrome OS-based machines accounted for between 5% and 10% of the Taiwanese computer maker's U.S. shipments in the fourth quarter, company president Jim Wong told Bloomberg Businessweek in a recent interview. Overall shipments dropped 28% over the same period, suggesting a weak climate for Windows PC sales.

"Windows 8 itself is still not successful," Wong told the magazine. "The whole market didn't come back to growth after the Windows 8 launch, that's a simple way to judge if it is successful or not."

His account matches up with what we've seen recently from the PC industry's other key participants. For example, Intel issued uninspiring first-quarter earlier this month, continuing a trends that's seen the chip maker's revenue and profit decline over the past year.

Microsoft (NASDAQ:MSFT) hasn't fared much better. Windows division revenue came in at $5.9 billion, a 24% year-over-year increase. Trouble is, that total is more than $1 billion short of Microsoft's last major release: Windows 7 helped push division revenue to $6.9 billion in fiscal 2010's second quarter.

Mr. Softy's backers will rightly point out that 60 million Windows 8 licenses have been sold to date. And yet license commitments don't equal quantifiable PC sales. We've no idea how many "licensed" Windows 8 PCs Dell, Hewlett-Packard, and their peers have sold as of this writing.

Which brings us back to Acer. If Chromebooks are selling well enough to account for more than 5% of of sales -- enough that the company is considering taking the device to new markets -- then there has to be a growing base of customers who use it as a PC alternative.