I'd say build the relationship. Remind them (politely) that they implemented the idea "you discussed in the interview". Don't suggest they stole it. I'd think t

I'd say build the relationship. Remind them (politely) that they implemented the idea "you discussed in the interview". Don't suggest they stole it. I'd think this is an opportunity for you to find out how it went/how it's going, decisions they made along the way, etc. You'll always know this was your idea, but it's just an idea. You'll have many others, probably better ones. This start-up might still fail. If they fail, would you be thinking "they had my great marketing idea and they failed". If they succeed, do you really think it would be because of your idea? I wish I had this amount of distance from my own ideas, but I know I don't. It's much easier when looking at someone else's ideas. But all in all, I'd say this is an opportunity to find out more, and maybe even come up with another idea based on the discussion you have with the company. It's up to you if you decide to share that further idea or not ;)