Enterprises I visit mostly Win7. App development obviously MacBooks commonplace, often with Windows VMs. Recent project with a university met quite a few students and staff still struggling with Linux but serious computing projects and students more likely Windows/VS and/or Apple gear (Apple offer healthy educational discounts to higher education, software is often free or heavily discounted to students).
Open source projects I'm involved in tend to be more about all three Android, OSX and Windows - not much pure linux - old school gpl definitely seems to be on the decline) - got an email this morning from a collaborator (half my age!) who'd already installed this community version of VS and on with porting code - a hobbyist thing he'd been stuck on VS2010 not wanting to pay for the latest version and Express lacked key features. Its not uncommon to use a MacBook when out and about but a desktop running Windows to do the heavy lifting back at base so coffee shop observations don't tell the story you need to actually speak with the users.
I agree MacBooks are popular with developers, more so in USA than UK but I've met many devs in their 20s using VS and XCode and Ellipse and I'd be surprised even in the 20s demographic if the proportion is over 40%. I think the fact you've not met any young devs using VS proves your personal experience is rather skewed from the real world.