I'm having the same problem right now.... Ive tried writting notes and everything.. it don't work.. but she is one of my many ex's as well. . so that might be one reason.. but I still want to be a part of her life, I still love her... But as a friend, you cant force them to stay around but you might need to back up and see how they react when you don't really hang around them
I think the best thing to do is to review yourself and see what you might have changed to make your friend talk to other people more. If there is nothing you have changed, this means that your best friend wants to make other friends and I guess there is nothing wrong with that. You can also ask her/him why they are not talking to you the way they used to.
Give them room. Make new friends of your own! No one likes a clingy friend they can't breathe around, so don't complain to her about it. You could ask what's going on, but be prepared to take your answer like a woman ;) keep ur chin up--it may not be as bad as you think
I am having a problem kind of like that, my friend got a new boyfriend and fell off the face of the earth! you will just have to give him/her time. They are just into something new and it is exciting so they really want to spend all their time with the new person. They always go back to their true friends.