If I were you, I would say something like
I have mistyped my ID (e.g. 12345) and pressed enter instead of backspace,
and I was dumbfounded to find that the page loaded and found
the location of a stranger who has the ID next to mine (e.g. 12346).
Being able to track someone without their permission
seems to be a security problem, as someone that knows me on Facebook
with a small bit of IT knowledge would be able to guess my ID without me knowing.
Basically, say you found it not by curiosity but by accident. Also send it pseudo anonymously (e.g. don't use your real id, and mail using something like email@example.com).
Check your mail before sending and maybe make someone you know proof read it.
Read it as if you were anger personified, that might help you smooth it so that you don't let someone that started his day by banging his toe on the bed post let out his anger on you.
If they don't answer, or don't do anything about it for some time say (maybe after a month or 2, as this is a medium security problem) that you would like them to do something about it or you will try to warn other users about this issue. If they still do nothing, do it, but be warned, they might not like it. see this zamfoo case
Insist that if you found it as trivially as this, someone worse might be using for nefarious purposes, and other users might have stumbled on that and be concerned too.
Use common sense so that you appear as a concerned user that stumbled on something weird or a friend of that user, that works too. A little "untruth", a lot of calm and politeness goes a long way to test the waters. If they seem friendly enough, you might say that you are competent (if you are) and can help them track the problem.
If you help them, and they are friendly, you might want to ask them if they want you to actively check other potential problems. (If they have great experience with you that might help you get a job (they can talk about you, how good (friendly but professional) you are etc.. to other people that might want you. Or just serve as a reference), or some friends.)
Also that's an occasion for them to get free advertisement, if they react well, you will probably be inclined to talk about them to people who might be interested.
Whatever you and them do, keep calm, don't escalate quickly, understand their point of view, and DO NOT APPEAR AS A THREAT (1)
If you look like you can damage them more than you would help, it is the quickest way to put them on the defensive, and get lawyer threats/a real case if you did something stupid and didn't cover your grounds.
(1) That works in most cases, not only when talking about security, but also with more or less angry people (coworkers, bosses).
The only time you may want appear as a threat, is if you are threatned by someone/something confident nothing will oppose them ever (e.g. a very angry dog), calmly walk towards them showing no fear (even if you brown your pants), they will probably start to bark more, but they will slowly back up, and let you pass (or kill/maul you, but if you fled that would be the same).
ps: Be critical of what I say, I'm a dumb human, and I do not own absolute truth, if something appears better, ponder the ideas, do what seems best and see what happens, learn.
(Also feel free to propose edits if that seems too unstructured/long/rambly, I do not bite.)