p.s. northmancpolak - i'm a she, not a he! ;)
OK, it wasn't obvious before, but that affects the translation as well.
I think i accidentally deleted you as a friend on facebook
But that's a completely different statement to your first one. To which the answer could be:
Myślę, że przypadkowo usunełam Cię z przyjaciół
(or you could change the bit in bold to say z listy kontaktów
, i.e. "from my list of contacts" instead)