Context. Redundancy in language is a wonderful thing. Let's take a noun, "Picture" Also, I should've said "decline" rather than "conjugate," apparently. Goes to show I don't know any language that well.

So, (leaving out the rest of the sentence for clarity, and just picking out the word "picture")
"You've been tagged in a picture" -> kuvassa
"Someone liked your picture" -> kuvastasi
"Someone tagged in your picture" -> kuvassasi
"Someone commented on your picture" -> kuvaasi
"Someone shared your picture" -> kuvasi

...etc :-) The messages all make sense despite the mis- or undeclined noun, because "like" and "picture" make it pretty obvious what happened, or "share" and "picture".