The map is created with data from Openstreetmap, which is a free map (data) created by users like you. Anyone can contribute to Openstreetmap. Take a look at the Terrengsykkel-guide (Norwegian) or Mountain biking (English) for a short guide how to contribute adding mountainbike trails to the map.
The MTB map can be used as an alternative to the Mapbox-map on the Strava web page. Just add the link button below to your bookmarks, and then click the link when you are browsing the map on Strava.
Note: May not work in all browsers. Seems to work in Chrome, Safari, IE and Opera, but not in Firefox or in iOS. If it still doesn't work, try changing the map style to Satellite map on the drop-down.
Strava – mtbmap ← drag this link to your bookmarks, or right click - add to bookmarks.
Alternatively, copy the code below and add it as a bookmark link.
MTBmap.se - A swedish MTB map covering all of the Nordic countries.
MTBMap.org - Another Norwegian MTB map
mtbmap.cz - MTB map of Europe
Israel MTB Map - MTB map of Israel. Similar style as MTBmap.no.
Openmtbmap.org - MTB and hiking maps for Garmin devices. World coverage.
The map is rendered in Maperitive, which happens once in a blue moon, or once a week'ish in certain areas if there is a good reason...