Facebook announced back at F8 - its annual developer conference - that it would be working on tools to make users spend less time on their platforms. Not because they want users spending less time, but to allow users to have a life, and do other things. This is a trend that Google and Apple had started at their developer conferences over the summer as well. Now, Facebook and Instagram's tools are rolling out. So you can see how much time you actually spend on either platform and limit your time on those platforms.
According to the blog post on Facebook's website, you can access these new tools by tapping on "Your Time on Facebook", and in Instagram's app by tapping on the "Your Activity". On both platforms, there is a dashboard that will show you the average time for that app on that particular device. At the bottom of the page, you can set a daily reminder to alert yourself when you've reached a certain amount of time in that app. Now that in itself won't keep you from opening the app again and spending more time on it. But you can go into notification settings to access the "Mute Push Notifications" and then turn off notifications once you have reached your limit. That will keep you from getting a notification and opening the app to see what's going on. This is also a good feature to use for those times when you need to focus.
The whole purpose for these features is that tech companies are attempting to get everyone to put down the phone and focus on real life instead of spending all your time on your smartphone. This is part of the Digital Wellbeing initiative that Google started first, and was followed by Apple and Facebook. And it should help you focus on other things than just spending time on Facebook.