How did this become my job?

Where do you draw the line between graciously offering to help and taking on things that are really not your job?  It's a problem I've seen over and over again in my career, and it just happened again today - being given work that's totally outside my job scope, just because there really isn't anybody else on the team whose job it is to do it.  Sometimes it's good to do it as a favor, but sometimes it's something that will never be useful to me, and just suck time and energy away from what my team is supposed to do. 

There was another incident recently where a friend was asked, as a job candidate, what their experience is with negotiating with executives.  As a community manager?  The hiring company clearly has no idea who we are and what we do.  We may be comfortable in that arena, but that doesn't mean it's an appropriate duty for what is typically a jeans and t-shirts role.