I think it needs to constantly reinvent itself to stay relevant and trendy. If it does not, then yes it will die out to be replaced by something someone hasn't thought of yet on a device that hasn't been invented yet.
I don't think Myspace was ever big enough to go the distance. Facebook is huge for now, but nothing lasts forever. I'm sure it will be taken over by a newer faster form of communication eventually.. Maybe telepathy because I'm not sure how much quicker it can get than a Facebook mobile app!