If I had to pay for Facebook I'd say fuck it and use the phome which I already pay for. If Quickbooks started free then asked me to pay I'd pony up because it not only saves me time and the monetary value of my time but it also saves me on hiring someone to do the work Quickbooks does for less. That's a real problem.
Another example happened to me just today. I'm heading up a project to start A/B testing our marketing efforts where I work. Right now I signed us up for a free Mixpanel account that I'm using to kind of start showing the team hard numbers before actually buying anything. This morning I had a meeting with someone from Optimizely which actually does exactly (literally exactly) what we want to do better than Mixpanel. I was shown how the service works and got a nice run through and at the end of the meeting I was dying to buy their platinum plan because of how much time it would save me personally, how much money it would save our company, and also for what the product itself does. My point here is that you don't see that in the B2C space. Most users would complain if a Facebook disappeared overnight but they sure as hell wouldn't whip out their wallets to keep it alive like B2B customers do when they like a service.