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Shows active 1 minute ago for a long time, what it means on fb app?

Sometimes i can see on mobile fb app that some of my friends are staying active 1 min ago for about 20 mins or longer, but when i open a chat it says they are active now, how to explain this? Does this could mean they are not browsing a fb but only chatting on msg?
Its hard to quess if they are really online, because on browser fb shows another time not 1 minute.
Whom to believe?
And i know this might happen when someone turns off chat, but im sure they didn't.
Shows active 1 minute ago for a long time, what it means on fb app?
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