The three co-founders are among the youngest members to join the "Giving Pledge."

I hate to break it to you, but a $10 million donation to increase Harvard's endowment doesn't have nearly the same impact as a similar gift to an institution aimed at social betterment.

Many companies are making important contributions, but CEOs have less leeway than ever to make bold investments in their communities. Due to growing pressures from short-term shareholders and increasing regulations, companies have been severely constrained in making targeted, long-term investments that drive social change.