So I've been using the Strava data quite a bit recently. I knew the service from before, but then it was quite empty. The tip came from our übermapilliariator Filip when I was making too much notes mapping a nearby forest.
Strava for forest trails
I have mapped a lot of trails in Flemish forests. We're a densely populated piece of land, with very little forest (in fact, our environment minister literally said that "the purpose of a tree has always been to be cut down"). But even here, I have hardly ever visited a forest where all forest paths were mapped.
It requires local surveying as paths below trees are completely invisible, and we tend to do a better job mapping stuff you can see on sat pics... But even when you do go out to the woods, the resulting GPS tracks can be of bad quality. Strava to the rescue! Several million trips by hiking and biking-nerds are mashed together to give a clear indication of where people run and bike.
The easiest way to use it, is with the [Strava ID editor](strava.github.io/iD/), which comes preloaded with the layers you need. I often switch of the satellite imagery to improve visibility of the tracks. This ID version also contains the Slide tool, which lets you adjust geometry to the available tracks. I haven't had very satisfying results with that myself though. In Belgian forest, you can basically zoom in anywhere and find missing tracks. (For JOSM instructions, see the wiki)
Strava and surveying
Of course, you still have to combine this with some satpic reading skills, other sources and/or local knowledge. For example, when Strava, Wegenregister and Groteroutepaden GPX all point in the same direction, you can be pretty sure there's a path present.
I did spot some situations where people seem to be running straight through a meadow where no path is visible. And the standard view does not take into account time. Sometimes, clear changes are visible over time, see this experiment. So just looking at the global heatmap might get you mapping former paths.
Strava in Osmand
If you don't have other sources, or just want to go hiking somewhere you suspect mapping is incomplete, you can add this layer to Osmand. It will help you find paths with bad geometry, and help you find unmapped paths.Vague lines on the map, combined with a visible trailhead can be enough to verify the existence of the path. So you can add much more paths with just one survey. Note: I hid all polygons and road details on my view, which helps keep the map readable.
In the tradition of the app, the feature is well hidden. First of all, you need to have the "Online maps" plugin enabled. This is just a setting, no downloads required. Standard available layers include "Microsoft Earth" satpics and online OSM maps.
Strava isn't standard. To add it as a layer, you need to open the "Map source" menu, available under map settings. Scroll down till you find "Define/edit". The URL example is with blue lines. You can find more about this URL on the wiki
Now your standard Osmand map is replaced with some blue lines. Great! Re-open the Map Source to get your "Offline vector maps" back. Now you can add the Strava layer as an Underlay or Overlay map. In the example above, I used it as an underlay with the basemap completely opaque. Forests (and other polygons) were switched off - but that does make for increased visibility.
(Note: I already contacted the Osmand Google Group with a feature request to make adding custom tiles just a little easier to use)