The most popular charity in America is United Way. The second most popular charity is Fidelity Charitable. If you haven’t heard of it, that’s because Fidelity Charitable isn’t really a charity at all, it’s what’s called a “donor-advised fund” run by the investment company Fidelity Investments. Donor-advised funds, or DAFs are growing. They control more than $70 billion in assets. But it’s not clear how much of that money actually makes it to charities.