Why can i only see someones facebook if theyre online?

[Sienna]

I cant see someones facebook, its like they blocked me but when they go online i can see them and click on their profile. I know this person didn't block me because ive asked people to check if they can find them on facebook and they cant, but then this person goes online and its like their facebook is back, why is that happening? Is there like a weird privacy setting that only allows people to see your facebook when you sign in?

[Sinterklaas]

They're most likely deactivating their profile every time they log out.