To begin with, I would seek to gain a comprehensive understanding of the issue at hand—specifically, the extent of the decline, the timeframe during which it occurred, and the geographical areas or demographics affected. I would analyze the data to identify any discernible patterns or irregularities. Following this, it would be crucial to ascertain whether this is a technical issue, such as a bug or outage, which would necessitate the involvement of the technical team for prompt resolution. If it appears not to be a technical issue, I would gather some market intelligence. Have there been any recent public relations crises that might influence user behavior, or significant actions from competitors that could affect our logins? Additionally, have we implemented any changes to our login process that could have discouraged users? Next, I would organize user research to gain insights into the situation from the user's viewpoint and collaborate with the product team to investigate any underlying technical problems that may not be immediately visible. Ultimately, based on the insights gathered, I would develop a strategy to tackle the issue. This could involve collaborating with the PR team to address any external perception challenges, working with the product team to resolve usability concerns, or partnering with the marketing team to enhance Facebook's visibility and appeal to both new and existing users.