Well I have found it happen sometimes more so at times as I have a personal fb account and a work fb account and unless I'm logged into the same account on the fb app and fb via the web browser then I get a blank web page our blank fb app page.
So go to fb on the phones browser and log in and click remember me and log in via the fb app and it should all be sorted and once you've done that go to the phones home screen and then long press on the screen to bring the options press on the folders option and then look for facebook contacts click on this and you will now have a folder with all your fb friends contact numbers if they have put it on the site.