Yesterday, my (samsung phone) facebook page changed to, well, something stupid looking, childish, primitive, difficult to use. I want to go back to what i had before, similar to what i get in my laptop. What happened ? What can i do ? 

Facebook probably did yet another "user experience enhancement" - and you're probably stuck with it.

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There are sites that you can download older versions of the Facebook app from.
You could try to find a version with a UI you like and then set Google Play to not auto update the app.
But you might find things stop working properly in time.

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Facebook changes interface to best suit its audience and their feedback. Stupid primitive and childish sounds like they are on the right track.

How old is your Samsung phone? I don't know about facebook but lots of websites are updated to newer technologies which only work with newer versions of browsers.

I suggest have a look at your fb page on a new Samsung phone and look if it looks "normal" on the new phone.

If yes, they try to update your phone. If you can't update it then look for a different browser on the same phone or a new phone.