OK...time to put my 2 cents out, as everyone else has.
First, of course I was pretty shocked when the news first came out (though a lot of people seem to have seen it coming, I didn't, which sounds about right where I'm concerned). And based off of the original blog post, and the fact that they've all known each other for years, I'm pretty sure they can remain friends with one another, cause there'd be nothing worse than falling out with people that you've been best friends with for years (been there, done that. It wasn't a nice experience).
Yes, the band's lost two of its original members. But there are bands out there who are still going even though they've been through so many line-up changes (Megadeth comes to mind), and I'm sure Paramore can keep going too, as long as they keep their drive and their passion. And I'll be enternally grateful to them as well: Brand New Eyes was possibly one of my fav albums of 2009, and since it came out not too long after my big brother passed away, I felt that it perfectly mapped out how I felt in the last year, from outright sorrow and anger, to finding the light again and learning to look towards the future with optimism.
Sure, some people have suggested that them saying all their problems were solved when BNE came out being BS, but that album came out over a year ago. A lot can happen in a year, to be sure. Josh and Zac clearly didn't make this decision lightly and a lot of people are upset, but there has to be a point where you be selfish for a moment and think of your own needs.
I wish them all the best for the future, not just Josh and Zac but the rest of Paramore too. And with the support of your fans, I'm sure there will be good times ahead.