I see this app as more like the Notes, iMessage or Freeform apps. There are tons of apps out there that do XYZ better, but Apple wants to ship a polished version that does 90% of everything the average user needs. It accomplishes three things (in my eyes):
1. It helps grow Apple's ecosystem by covering just enough ground to make third-party alternatives less necessary for most users.
2. It reduces one of the major "sticky" points that keep people in Facebook's own moat. Events and Marketplace are the two reasons I still use Facebook.
3. It encourages competition from the people who want to do that last 10% better than Apple's apps, raising the baseline and hopefully forcing innovation as well. Those apps lead to more App Store revenue, so, cynically, it's a win-win for Apple.