-- Stranger Danger
"Stranger danger" is listed here as a courtesy to public expectations, nothing more; the actual incidence rate of stranger abduction and assault is quite low, and the very nature of social media means that it's challenging to trace the movements of someone you don't actually know – you need to be virtual friends (or at least friends of friends) before you can begin following a trail of breadcrumbs toward your target.
More realistic than your teen being hunted down by some random predator is your teen being teased, hazed, or even physically bullied by a person or persons with a specific grudge against him or her. Inasmuch as social media and location-based services make it easier for us to learn more about our friends, it also gives bullies more ammunition to use against their targets, just part of the much bigger trend of bullying (particularly emotional bullying) intensifying in the digital age.
Related to bullies (but more specifically relevant to kids in their late teens) are stalkers – romantically obsessed former flames (or would-be flames) who latch to the objects of their obsessions and refuse to let go. For these folks, check ins can serve as a how-to guide to create chance encounters, keep up with potential romantic "rivals," and generally make their targets' lives miserable.