I think FB is mostly public information isn’t it? So why would having software to give you updates be illegal? It’s no different from getting notifications from any other public website.
Read you FB T&Cs
m.facebook.com/about/privacy/update#
Look at How is Your Information Shared under Data Policy
“Sharing on Facebook Products
People and accounts that you share and communicate with
When you share and communicate using our Products, you choose the audience for what you share. For example, when you post on Facebook, you select the audience for the post, such as a group, all of your friends, the public or a customised list of people. Similarly, when you use Messenger or Instagram to communicate with people or businesses, those people and businesses can see the content you send. Your network can also see actions that you have taken on our Products, including engagement with ads and sponsored content. We also let other accounts see who has viewed their Facebook or Instagram Stories.
Public information can be seen by anyone, on or off our Products, including if they don't have an account. This includes your Instagram username, any information you share with a public audience, information in your public profile on Facebook, and content you share on a Facebook Page, public Instagram account or any other public forum, such as Facebook Marketplace. You, other people using Facebook and Instagram, and we can provide access to or send public information to anyone on or off our Products, including in other Facebook Company Products, in search results or through tools and APIs. Public information can also be seen, accessed, reshared or downloaded through third-party services such as search engines, APIs and offline media such as TV, and by apps, websites and other services that integrate with our Products.”
