There are ways to calculate ROI from advertising in all types of media. They will be somewhat less accurate than the guy who says, I got 10 clicks and 2 bought my product. But even that is somewhat misleading as some people who click only become a customer later.
How you measure ROI also depends on what your marketing objectives are. Different businesses have different metrics that help measure success.
It's been stated before on this board that huge brands are spending lots of money on Facebook. Well, that's because most of their marketers have experience in consumer products marketing. In that classic discipline, in the world before the web, there was rarely an easy way to instantly correlate ROI with marketing activities. Consumer products companies and Brand Managers (gurus?) are comfortable with such a world and have many ways to measure the effectiveness of their marketing.