To begin with, I would carry out a thorough analysis to determine the reasons behind the decline in metrics. I would take into account all potential factors, such as algorithm updates, seasonal fluctuations, changes in user behavior, technical difficulties, or modifications to our campaign strategies. After pinpointing the likely causes, I would utilize Facebook's analytics tools or other third-party platforms to gain a more detailed understanding of the performance. My focus would be on demographic information, engagement trends, click-through rates, and any available user feedback. Once I have collected all the relevant data, I would formulate a strategy to tackle the issue. This may involve conducting more experiments to identify effective approaches, adjusting event promotions based on demographic engagement, or enhancing event content to boost user interaction. It would be essential to implement follow-up measurements to ensure we are back on track. Finally, to avoid similar issues in the future, I would recommend establishing a regular monitoring system to quickly detect any significant changes in engagement or user feedback, enabling us to respond promptly if needed.