Note that Friendly AI (if it works) will defeat all (or at least a lot of) x-risk. So AI has a good claim to being the most effective at reducing x-risks, even the ones that aren't AI risk. If you anticipate an intelligence explosion but aren't worried about UFAI then your favourite charity is probably some non-MIRI AI research lab (Google?).
As far as I remember from LW census data the median date for predicted AGI intelligence explosion didn't fall in this century and more people considered bioengineered pandemics the most probably X-risk in this century than UFAI.
You know the drill - If it's worth saying, but not worth its own post (even in Discussion), then it goes here.