Here are some things Strava may reveal
Whether you run, swim, ski or cycle.
If you tell it, what bicycles you have.
Who you go out on a run or ride with
When you are away from your house
Where you commute to, and when
Your fitness, and whether it is getting better or worse.
When you travel, what TZ, etc.
The fact that you can infer nation-state secrets is an interesting escalation. Currently it’s the heatmap which is getting the bad press, which is part of the dataset that Strava offer commercially to councils. FWIW, the selection bias on Strava data (male roadies or mountain bikers) means that its not that good. If someone bought our local data, they’d infer that muddy wood trails with trees and rocks are what the city needs. Which is true, but it doesn’t address the lack of any safe way to cross the city.
What is interesting about the heat map, and not picked up on yet, is that you can potentially deanonymize people from it.