Is someone using my account on FB?

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yesterday, me and my cousin are chatting on FB and the next day, I saw mediafire links that "i" commented?! i didn't comment that!? i was freaking out but i thought maybe it was just a mistake
and the next week/day there's more and more! mediafire links that i commented!! I was so freaking out to that and i saw i commented a Spam! I'm very shocked right now. if i'm hacked then please tell me how i can avoid it? if i'm not hacked then..That will be weird! No one knows my account! even my cousin! my cousin is living in another province. WEIRD..and i do not post mediafire links on FB!
and when i clicked that mediafire links, some are errors and some are random things!.
Please Help
for more details:
These are the Random links that "i" commented (which i didn't really commented it)
http://www.mediafire.com/?o1co63pj37mgxz8
http://www.mediafire.com/?mrcbj39bg1x6c1z
http://www.mediafire.com/?taloo44ac1b8luy
http://www.mediafire.com/?o1co63pj37mgxz8
http://www.mediafire.com/?lxyz9ne0zasy7e9
Weird!