Hi, as requested:

(note: my comments marked with {{ }} )

(note2: there are screenshots name gwibber-01 to gwibber-06 in PNG format, please see attached zip file)

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jc@Home-desktop01:~$ gwibber

** (gwibber:12253): WARNING **: Trying to register gtype 'WnckWindowState' as enum when in fact it is of type 'GFlags'

** (gwibber:12253): WARNING **: Trying to register gtype 'WnckWindowActions' as enum when in fact it is of type 'GFlags'

** (gwibber:12253): WARNING **: Trying to register gtype 'WnckWindowMoveResizeMask' as enum when in fact it is of type 'GFlags'
No dbus monitor yet
Updating...
Updating...
ERROR:dbus.proxies:Introspect error on com.Gwibber.Accounts:/com/gwibber/Accounts: dbus.exceptions.DBusException: org.freedesktop.DBus.Error.ServiceUnknown: The name com.Gwibber.Accounts was not provided by any .service files
ERROR:dbus.proxies:Introspect error on com.Gwibber.Streams:/com/gwibber/Streams: dbus.exceptions.DBusException: org.freedesktop.DBus.Error.ServiceUnknown: The name com.Gwibber.Streams was not provided by any .service files
/usr/bin/gwibber:68: GtkWarning: gtk_container_add: assertion `GTK_IS_CONTAINER (container)' failed
  gtk.main()

{{ At this point, I tried adding my Facebook account }}

** (gwibber-accounts:12345): WARNING **: Trying to register gtype 'WnckWindowState' as enum when in fact it is of type 'GFlags'

** (gwibber-accounts:12345): WARNING **: Trying to register gtype 'WnckWindowActions' as enum when in fact it is of type 'GFlags'

** (gwibber-accounts:12345): WARNING **: Trying to register gtype 'WnckWindowMoveResizeMask' as enum when in fact it is of type 'GFlags'

{{ See screenshots gwibber-01 to gwibber-05 }}

Updating...

{{ I clicked the "Close" button since the "Add" button is missing and tried again }}

** (gwibber-accounts:12553): WARNING **: Trying to register gtype 'WnckWindowState' as enum when in fact it is of type 'GFlags'

** (gwibber-accounts:12553): WARNING **: Trying to register gtype 'WnckWindowActions' as enum when in fact it is of type 'GFlags'

** (gwibber-accounts:12553): WARNING **: Trying to register gtype 'WnckWindowMoveResizeMask' as enum when in fact it is of type 'GFlags'
Updating...
** Message: console message: http://static.ak.fbcdn.net/rsrc.php/z2JGI/hash/cd0s5yom.js @9: ReferenceError: Can't find variable: hide

{{ The missing "Add" button showed up this time }}

Saving...
Could not identify preference: username
Could not identify preference: session_key

{{ I deleted my Facebook account again and tried re-adding it }}

** (gwibber-accounts:12656): WARNING **: Trying to register gtype 'WnckWindowState' as enum when in fact it is of type 'GFlags'

** (gwibber-accounts:12656): WARNING **: Trying to register gtype 'WnckWindowActions' as enum when in fact it is of type 'GFlags'

** (gwibber-accounts:12656): WARNING **: Trying to register gtype 'WnckWindowMoveResizeMask' as enum when in fact it is of type 'GFlags'
Updating...

** (gwibber-accounts:12714): WARNING **: Trying to register gtype 'WnckWindowState' as enum when in fact it is of type 'GFlags'

** (gwibber-accounts:12714): WARNING **: Trying to register gtype 'WnckWindowActions' as enum when in fact it is of type 'GFlags'

** (gwibber-accounts:12714): WARNING **: Trying to register gtype 'WnckWindowMoveResizeMask' as enum when in fact it is of type 'GFlags'
Updating...
** Message: console message: http://static.ak.fbcdn.net/rsrc.php/z2JGI/hash/cd0s5yom.js @9: ReferenceError: Can't find variable: hide

{{ Again, the missing "Add" button showed up }}

Saving...
Could not identify preference: username
Could not identify preference: session_key
CouchDB ID collision encountered, deleting old account
INFO:Gwibber GNOME Client:Gwibber Client quit
INFO:Gwibber GNOME Client:Gwibber Client closed
jc@Home-desktop01:~$ 
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Something happened that did not happen before.  As I mentioned earlier (comment #11 in this thread), I maximized the Accounts window so I can see the "Add" button.  However, during my first attempt via terminal, the "Add" button is missing regardless of how big I resize the Accounts window.  And then, when I attempted it again, the missing "Add" button shows up just fine - which I did twice and the button still showed up, as you can see in screenshot gwibber-06.

Interesting also, screenshots gwibber-05 and gwibber-06 are different.  Again, gwibber-05 is _after_ a supposedly successful authorization of my Facebook account.  But after seeing gwibber-06, I think the initial attempt was a false alarm, something or somewhere along the way Gwibber is telling the user that it was successful by showing "Facebook authorized" (gwibber-05) when it should be "{Facebook name} has been authorized by Facebook" as shown in gwibber-06.

Sadly, I can not remember which of the two (ie gwibber-05 or gwibber-06) showed up during my explanation in comment #11.  Although I am more inclined to say it was gwibber-05, since if screenshot gwibber-06 was the one that showed up back then, obviously there is no need for me to resize the Accounts window bigger just to see the missing "Add" button.  (I hope you're following the flow and logic..)

Some notes:
Gwibber app is _already_ authorized on my Facebook account when I was doing these tests.  If I remember correctly, after signing-in to Facebook, Gwibber will show Facebook authorization requests like "authorize to post in your wall" (or something) before Gwibber shows the "Add" button as shown in gwibber-06 screenshot.

Hope that helps.