The (later Nokia Store) was shut down permanently in 2014. Even if you factory-reset your E90, you cannot access the app repository. Your only source is third-party archives like Archive.org or Symbian-freak.com.
Snaptu is a popular third-party app that provides a Facebook client, along with other social media services. The features include: facebook app for nokia e90
: Once patched, you can navigate to m.facebook.com or the ultra-lightweight mbasic.facebook.com . The E90’s large internal 800x352 screen is particularly good for these mobile views. 2. "Facebook for Every Phone" (Java App) The (later Nokia Store) was shut down permanently in 2014