Microfish_1: surely that only includes hours played from games launched via the launcher, and not the old method of downloading the offline installer or launching outside of the launcher?
It includes how many games I own, and thus when queried regularly, allows tracking of when I buy how many games. Given sufficient friends (particularly those with few other friends), the "Popular with friends" feature would even allow tracking which games those are.
I'm not worried here about individual friends knowing these things (though 'micro-leaks' like the above "played for 48 hours" or "I claimed to be broke, but two days later added 20 games to my list" are bad in their own way), and obviously not about gog, but extracting private information from public profiles is big business, and this is exactly the type of information which can be pretty valuable, as it directly relates to money being spent. I cannot imagine anything other than that this is being done by several parties for the entire gog user base, or at least for any username who's ever posted in the forum or is connected through public friend chains to anyone who has done so. And then of course, if you use your username across multiple sites, this information gets connected to everything else which is known about you. It's the chief reason why gog's launch of "public by default" user profiles was such a bad idea.
(I realize a similar thing is also possible through the public wishlist feature; which is why the games on my public wishlist are mostly marginal wishes I might someday get around to looking at, and a game being removed from my wishlist is as likely to have been bought as have been deemed not worth buying ever - stuff I really want, I just buy.)