"It's difficult to say. About 10% of personal computers run Linux or OS X. But then there are servers and Internet-connected devices to consider."
I suppose I agree that such estimations are a bit difficult, but you're certainly quite a bit off here. The actual 'market share' (quarterly sales, etc.) figures for Apple haven't been below 10% for the last decade or two. That's not even considering actual IN USE unit percentages for each OS (for example a typical Windows unit might become a cash register or have Unix loaded on it). A more realistic figure would probably be above 20% (and if we weren't counting boxen in corporations, managed by IT staff, we'd probably be talking 40% or more).
As far as vulnerability, the test you've outlined only shows if you have a 'vulnerable' version, not that you're *actually* vulnerable. For example, on OSX, unless you've turned on command-line remote access or are using some software which is externally available that implements Bash, you'd only be vulnerable to LOCAL attacks (ie: someone sitting at your machine).
So, while this is certainly a serious threat, it's also important to keep it in perspective.