Clarification: I'm not a native English speaker. I've been gaming online for the past ~15-20 years, and it feels like there has been an increase in this kind of response in the past few years. Now, I know that it's not always serious: people started using it as 'no way' or meaning 'wow, serious?' recently.
BUT what has taken me aback is the frequency at which I've been seeing 'stfu' as a response to something I either meant seriously, or maybe said it in a light-hearted manner, but not necessarily as a joke - often as a first exchange. To me it still literally means Shut The Fuck Up, and whenever I see it, my immediate reaction is that the person on the other end actually meant it exactly as rude as I have always perceived it all my life.
Would you not take this as an offense, provided it's the first interaction between you and a completely random person? Would you immediately feel they are rude, neutral or straight out saying it very lightly? Is the usage really changing? Do you think they actually mean 'shut up', and they just add the 'f' because everyone else does it?
EDIT: Wow, thanks a lot for your angles! I forgot to mention, but let's assume that 'stfu' is the first and only response you see from this random person. I am also fine with dealing with different situations, I'm not primarily looking for advice (After ~20 years of online gaming experience, I can tell you I've seen most of it); I am mostly interested in your perspective: what vibe would you assume, based on only these four letters?
All I see is a zerg overlord.