Jumo is a similar idea to idealist but I think/hope jumo can do it better. Chris did a great job to help organize the movement supporting Obama during his campaign, if he can do something similar for a bunch of non-profits a lot of good could come from his efforts.
It is also a big deal because it is Facebook's co-founder who is doing this. Almost any company that he is launching would get press.
There's actually quite a good number many more than the site you mentioned. I put them in two basic categories.
1) Overly complex - you arrive at the site and you have a thousand options and don't know what the heck to do. They offer these options hoping to match you to the things you care about, but there's no motivation to sift through it all or click through a system of menus or search results. Examples: change.org, current.com, takepart.com
2) Too basic to expand impact - the barrier to entry is incredibly low so they can have quick impact, but they are hyper-focused and offer no other options than the cause they are working towards. Examples: freerice.com, kiva.org, charitywater.org
I will never fault the motivations of any of these sites, and any new entrant to this arena should be given the benefit of the doubt. I'm very interested considering the impact he and the designer had on Obama's campaign, and think that he could someone find that middle ground between those two categories I described.
http://www.idealist.org
Not sure why this is a big deal.