When has it been a common courtesy to pick up strangers on your way to work?

A courtesy, yes. But a common one?

(I'm actually genuinely interested - was this common at some point in history?)

In post-soviet countries like Poland it was (less so now, but still popular). At some point we even had "hitchhiker's booklets" with coupons you would leave drivers who took you, and they could then use in lotteries. Hitchhiking was a strong tradition in Central/Eastern Europe, and - in some parts of the continent - still is.

Apparently it is a thing in DC, known as "slug lines": http://en.wikipedia.org/wiki/Slugging

The idea is that by picking up people, you can drive in the HOV lanes.

It's common in San Francisco: http://sfcasualcarpool.com/

I am pretty happy that Lyft is betting on carpooling as that was the original vision that they had for Zimride (where Lyft was born) and I feel it sets a very clear distinction against the vision Uber has. Lyft is carpooling with friendly strangers, Uber is everyone's private driver (cheaper taxi).

The challenge with the free/common courtesy model is that casual carpools are just not getting enough cars off the road. Lyft is solving the two biggest challenges I see in carpooling 1) convenience of having the app route you to people who are close enough to your route and can be dropped off without adding too much time and 2) making you feel more comfortable picking up strangers. Adding money to the equation to encourage more drivers is fine with me as it reduces car traffic and pollution.

As I said, I have conflicting feelings about this. On the one hand, I very much agree with what you wrote, and the tangible benefits this model has. On the other hand, picking up random passengers really was about just being nice to another human being. So between Lyft and BlaBlaCar it feels like we're losing a little bit of humanity again.

To be clear, I'm not saying it's a bad thing - it's just that my alarm bells go off every time I see a business creeping into what used to be a money-free zone, so I thought I'd raise that concern.