A break up is not only hard to you and your now ex partner, but it's also hard on mutual friends. Who do they stay friends with? The person they've known longer, or the person they are closer to. Do they stay friends with the dumper or the dumpee. Who was the right one in the relationship?
Usually, the friends feel like they are forced to choose sides - and often enough, one of you, or both tells the friend or friends, "It's me or them, you can't be friends with both." Even if this doesn't occur, a lot of friends tend to lean to one person more then the other and while you might occasionally see that person in social situations, you're more of acquaintances then friends now.
While during the break up, you should surround yourself with good, loving, caring friends, and most imporantly loyal friends, don't force your friends to choose sides. Don't bad mouth your ex to mutual friends either. Bad mouthing your ex to mutual friends only makes you look immature, and might push them away. Making friends choose also pushes them away. Eventually, they will drift more to one side then the other, but don't have hurt feelings over it.
There will be fair weather friends. Those are the friends you should watch out for. While to them it might come off as loyalty to their friend, your ex, it comes off and down right disgusting and despicable. I have a friend who was in a three year relationship. Her (ex) boyfriend's friends, a couple, for three years treated my friend like she was one of their best friends. The second my friend and her boyfriend broke up (and it was an amicable break up), they shunned her as if she had done unspeakable things to their family. This isn't high school any more, if someone plays the game of BBFs on Monday and Enemies Forever for Thursday, don't waste your time with it. Don't even think about it. Better you should find out now, then when it's something more important. Would you rather find out after you had them in your wedding party?
Your true friends will stick by you, and that's all that matters.
Your real friends will be the ones, who despite you saying you want to stay at home and mope, are forcing you to get dressed and come out. That same ones who will listen to you bitch and moan over the break up for the next couple of months. They are the ones who will be there when you need them most. They will call you to check up on you.
Some of your friends might "go to the other side," then a couple of months later, sheepishly give you a call. they might apologize, because "it was weird," let them come back. But the ones who wrote you off completely, those, you can forget. Those type of friends, you don't need.