I remember reading stories a few years ago about how facebook employees used to spy on love interests. I'm sure there are better access controls in place these days, though.
However, if anyone at google or facebook gets caught doing that, it will have a hard time finding a job. At the NSA is probably encouraged behavior and I'm pretty no one is loosing his job over spying someone there, it's what they do after all.
Another one is for someone to "throw the net" and get information on several fronts (possibly including: credit report, bank info, emails, etc)