This may not be related to FBP. I have hadit happen my Chromebook when I just stay on a site without any interaction and Facebook was one of those sites. In my case it was a setting that put my chromebook to sleep when it regarded that it was idle for more than 10 minutes. Maybe you have a similar setting in Windows ?
Probably I'm wrong and just scrolling for ages would still keep your laptop awake, it maybe just mine being a tad empathic as face book can sometimes put me to sleep as well.