Have you noticed that segments of your social circle seem to be breaking off into smaller units? You can try to figure out why this is happening, but what's the point? Reshuffling happens all the time, and it's usually an organic and necessary process. If you get caught up in who's spending more time with whom, you're only going to drive yourself crazy. Just be happy that so-and-so is getting to know so-and-so better and leave it alone.