durandl: Why is it so hard to make a new account for a site / when you can basically log in to anything automatically? Especially when every browser nowadays can log you in anywhere you need to. But no, we need websites now, that do who knows what with our private data just to log into a site we find "cool". And with that we basically infiltrate our own privacy just because we are so damn lazy. I actually call this insane.
Because not all people are the same, I'll answer your questions drawing from my own experience. I personally (as I mentioned on my previous post) login via Google to academia.edu, a site that hosts academic papers, articles e.t.c, but in which one needs to login to download anything. Because I generally don't login to Academia all that often (only when I need to download a paper - which is mainly in the beginning when I do research to write my uni papers, and then only every few days - and then promptly log out), I prefer to login via Google because all I want to do is download the paper and get out. That is I don't want to invest in the service - I just want to get some stuff from there and then leave. Theoretically I
could have created an account even though I'm not going to be invested in the site, but given that it asks for my name and surname (other than the obligatory password and email fields), I might as well login via Google where I've put a fake name and surname. And another reason I don't want to create accounts for services I don't see myself getting invested in is that I have enough accounts already to various places, and I don't feel like juggling another one when I don't feel it's worth it.
Btw, now that I think about it, regarding the point I just said - that I don't create accounts for sites I don't feel I'm going to get invested about - that's probably going to be the case with GOG too - that is the new people coming will probably get the initial free games and then the ocasional freebies, but nothing guarantees they're going to buy anything, because that would mean they'd be investing on the site...
Regarding now the second part of your question, why having to juggle accounts, when one can automatically log in, I personally feel that automatically logging in is less secure that manually doing so (and for the same reason I also haven't saved my credit card number or bought wallet credit here on GOG). You might say "And yet you log into academia with Google!" (given that as I just saw from other posts, if someone hacked my Google account - knock wood! - , they'd get access to academia), but given I'm not invested there, the hacker wouldn't find anything he wouldn't already know from the Google account, and besides, as soon as I'm done with academia, I also log out from the Google account too, if I don't happen to be using it for anything else at that moment (e.g. Google Docs).
So, what I'm saying is that logging into somewhere with a third-party account, isn't necessarily (just) laziness, it also reflects a general reluctance to get invested in the service where the account has to be created. And that's why this action by GOG to add Facebook login button won't, now that I thought about it, guarantee more customers and more sales.
P.S.
Thank you for helping me get some extra insight on the matter by reflecting on my personal experiences.