Whether they directly used my solution or gave that problem to several interviewees and picked the implementation they liked best I couldn't know. But I certainly felt used. There's a certain popular piece of software I refuse to use to this day because of my experience interviewing with them (fortunately, they have competition now).

Eventually you get to a point where your gut will tell you whether an 'interview project' is a toy problem or something that might actually be useful.

That said, do be sure it's your code... pretty much by definition, if you accomplished it during an interview, they could accomplish it in the same or less time themselves freshly and legally... for all you know, they'd already written it, just hadn't deployed it yet, and were using it as an interview question precisely because they had just written it themselves and it was fresh on their minds. Given the prevalence of NIH syndrome, this would even strike me as the most likely explanation unless you have some sort of really solid proof it's your code... generally, even when a programmer should take somebody else's two or three hour work they'd rather do it themselves!