Ubisoft's medieval fantasy-ish combat simulator For Honor saw a massive player influx as a direct consequence of going free for a limited time on Steam. Many players may not have liked the game, as they deserted it just one day later.
For Honor seems to be living its own version of Flowers for Algernon, as the game is gaining and shedding players in rapid fashion. The game went from concurrent player counts between 2.000 and 5.000 players to 225.831 thanks to going free on Steam for a limited time. The change happened overnight, literally.
Fast forward 24 hours, however, and there are "only" 68,286 players at the time of writing meaning the game lost around 70 per cent of those players in less than a day. This can be easily attributed to players jumping at the opportunity to snatch a AAA title for free, giving it a shot and, finding it's not their cup of tea, simply discarding it.
Another possible cause is that For Honor is not exactly friendly to new players. Those who jump into PvP without playing the lukewarm single player first, or even those who did will often find themselves ravaged by the more experienced players. This in part due to For Honor being intended as a competitive game, requiring excellent mechanics and game knowledge in order to be up to par in combat.
Keep in mind that we are still in the first days of the game going free on Steam though, and numbers are normally hectic in just about every game that gets a major update, goes free or gets released. The numbers should be normalised in the period of the next week or two, at which point it might be worthwhile revising For Honor's current state.
It is worth keeping in mind that For Honor will require you to download Uplay as well. Another reason why some players may not be showing up on Steam's CCU stats may be that a lot of players simply decided to save some resources by not running Steam on top of Ubisoft's service.
That said, if you haven't grabbed it, you should. It's a free game, so the worst case scenario is that you never actually play it. If it does turn to be up your alley, you might want to hold off on spending money on microtransactions and consider actually getting a Standard edition as Ubisoft's Starter editions are notorious for their grinding requirements.