Idea stolen during an interview. What should I do?

Keeping this intentionally vague.

I interviewed with a series B company a couple of months ago. During the interview with the CTO I proposed a way that could effectively 3X their MAU. They were genuinely impressed by it, and stated they didn't think of that. Like it seemed natural, and I'm considered an expert in this field.

I ended up not getting the job, and not thinking much of it. Fast-forward a couple of months, and it's their new growth strategy. I got no credit for this, and not even a call back. Is there anything I can do?