I would refute by saying:
1. none of us has any idea how much of his wealth, if any, he gives away. And it's not really our business.
2. he's taken no vow of poverty. Quite the opposite, his standard line is that he'd vastly prefer being a decadent rock star (even if I suspect he's being cheeky when he says this). He openly enjoys his wealth, without apologies. He's no more a hypocrite for this than Bill Gates is for continuing to hang on to a few of his (once) tens of billions in wealth. Gates is an amazing human being for the same reasons that Bono is -- they're involved in these issues deeply, beyond any personal contribution.
3. when it comes to his most prominent cause, he's the first to say that charity is ineffective -- it's not money per se, it's policy, priorities, etc, that will ultimately make for a more just world. Governments collect taxes, period; the question is, how will those revenues be redistributed? For defense purposes? For corporate welfare? For subsidies that make us (in rich countries) lazy and content while locking out international competition? Or will rich countries see the benefit of enlightened self-interest -- will see that festering poverty, no matter how far away, threatens our security (for those who need practical rather than moral reasons to act). Will governments recognize that we've invested successfully in developing nations before -- so why not decently governed African nations now?
4. the impact of his efforts, his time, his persuasive ability, his wonkishness, his lobbying, etc, on behalf of the poorest of the poor are undeniable. With lots of smart, talented people around him, debt has been forgiven, aid for highly preventable and curable diseases has skyrocketed by the billions, drug treatments by the millions have been distributed, kids have gone to school -- all with Bono as the world's most prominent lobbyist.
There are a number of articles in sources such as the New York Times, Time Magazine, and others that detail the extent of Bono's political and policy involvement and his effectiveness as a strategist and negotiator, not just a figurehead.
I think it's appropriate and in Bono's best interest for there to be skepticism -- skepticism heightens personal accountability. Since Bono has created this role of humantarian for himself, he does have a higher standard. But I don't think the "hypocrisy" angle around his charitable giving is a successful line of attack. A more interesting, but subtle area of inquiry is whether Bono's campaigns have put enough focus on the tougher nuts to crack -- like truly opening up trade and creating a level playing field. Or whether these campaigns generate disproportionately negative views of Africa, which help perpetuate the cycles. Or whether there's enough focus on the Accountability issue (which is the second "A" in DATA). All these areas are worth a rich discussion. But the easy, FOXNEWS / Talk Radio style gotcha / outrage journalism really holds no water.