Someone that I'm following started following another person. They were already following a lot of people so I can't tell who the person they started following is. How can I see who they started following?
You would need to cache the person's data and compare the previous sets.
And even if you were able to do this, here are some questions that will come up
What would happen to those who use tools to automatically follow then un-follow based on certain criteria.
Which date should be chosen ?
What about those who start off on someone's list then move to follow status ?
Or those support company accounts who ask you to follow them for a period to send a direct message ?
Certain criteria would have to be met to define exactly what date qualifies as starting to follow someone.
If you visit the list of people I'm following: http://twitter.com/#!/rvn/following/people
The person on top is the most recent user that I followed @LottoPH, and that's how twitter lists them. The very first user I followed is at the bottom, @noi13.
My guess is that this is the same order for everyone.
I'm not sure there is a service that will send you notifications when someone you follow, starts to follow someone else.
There is DoesFollow which will tell if one person is following someone else.
There is also ReFollow which is a sort of follower management tool. There are many different ways to sort your followers and to see who the people you follow are following.