That's part of the point--people irrationally care more about the marginal puppy than the marginal degree of killer asteroid/supervolcano/nuclear war prevention. If you're one of the few people who care about making a small contribution toward preventing nuclear war than a large contribution toward saving a single puppy, the websites look much saner.

I'm late to the party, but rationally speaking...

Chance that a bunch of nice, sane, ordinary people can save the lives of lots of cute animals (or hell, humans)? Very good.

Chance that a bunch of crazy people with their crazy web site can stop aging, or better yet, stop an asteroid?

Slim to none.

In order to speak rationally about it, you would have to give the likelihood ratio you believe they could reduce the sum of existential threats by, however low that is; then multiply that by the utility you would receive from saving human civilization. Compare this to the utility you receive from saving a puppy, multiplied by the probability of saving that puppy. Compare the two numbers.

Rationality means thinking quantitatively where applicable, not qualitatively.

For those who want to learn more:

http://www.nickbostrom.com/existential/risks.html

http://en.wikipedia.org/wiki/Existential_risk