Some years ago, soon after the original iPhone took firm hold of the public's imagination, an iPhone-using friend of mine went through a messy breakup. He was the instigator and his partner was, well, the breakup-ee. In the following weeks, my friend's ex kept sending him creepy messages with more knowledge about his activities than any ex should have. The ex also would show up, unannounced, in situations an ex shouldn't know anything about. And that was just the beginning.
Through his iPhone, this crazy ex had gained access to my friend's activities and communications. This wasn't possible in the old feature phone days, and my friend thought that not making his life public on social networks would keep him safe from stalkers. A few freaked out phone calls, a bunch of settings changes, and one restraining order later—we all learned otherwise.
I was reminded of this story again recently after seeing this reddit thread (the original contents of which have since been removed). The poster had been watching his ex-girlfriend's whereabouts on her iPhone—not with some agreed-upon permission in Find My Friends, but from within her own iCloud account using Find My iPhone. The poster also admitted to changing the names of contacts in his ex-girlfriend's address book so that numerous text messages from her would go to him instead of to her friends. He would also change other people's contacts to match his own number so he could text, masking himself as another friend. He had originally set up her iTunes password and so was able to access everything about her—as her.