Why do people map their run and post it on social media?

They want to brag about it or inspire other runners. Some runners do change up their routes from time to time, so it may not always be easy to do that.

There is privacy to consider too, no one can view your profile unless you invite them or become friends with them. Also runners should be aware of their surroundings or carry some thing to defend them selves with and they should bring their phone with them. When I was running, I carried mace and my phone with me. 

I don't do that, and I can't really say what motivates others. But one possibility is they have some sort of goal. For example, they might be a member of a group that wants to run a total of 3,000 miles in 3 months. 

A lot of runners are proud of their activities and their milestone runs (such as putting a 13.1 magnet on their car). Posting ANYTHING personal on social media is a risk, but a lot of users limit who can see their posts. Some people just don't think before they post (and do open themselves up to harassment or worse).

Some people just like to share some (or nearly all) of their daily lives online. Some limit who can see such posts (just their friends or designated individuals). Yes, if the "wrong" eyes see such posts, it can set someone up for harm or at the very least, to have their house broken into while they are out on their run (or checked in at a restaurant/business or event). 

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Social media has definitely caused a lot of people to feel the desire or need to share a lot about their comings and goings, and daily activities. Most of the time, it's not a problem. Sometimes, it can lead to an unfortunate or tragic event (over sharing).

Runners are sometimes odd sorts (I mean, is it really meaningful to drive around with a sticker that says "13.1"?) but social media in general is a vast wasteland of abdicated personal privacy rights.

I read an article a few years back where a personal privacy expert said anyone who's been active on social media for more than 6-10 months has already left enough of a 'digital footprint' to be tracked and identified online for the rest of his/her life.

"Data mining" programs capture everything from the things you "like" to the phrases you use to the trackable info you accidently give (a comment like "I know this guy here in Denver..." for example).  Not as overt as a map, of course, but far more common and possibly more potentially "dangerous" as we move "forward" in data collection and tracking methods.

The possibility of being kidnapped flies right over their head. It's a personal achievement for them and they want to share it with the world, which is usually fine, but they seem to ignore the fact that their exact route was shared to anybody and everybody.