I think it's a problem of fb's server. I remember in somewhere it's said that fb xmpp server don't allow gateway server login in on behalf of user, but I really cannot find that page right now.

The following is the code I have, it works fine with me, FYI.

    public void syncUsers() {
        for (RosterEntry entry : conn.getRoster().getEntries()) {
            getBuddyManager().storeBuddy(new XMPPBuddy(getBuddyManager(), entry.getUser(), entry.getName(), entry.getGroups(), entry));
            if (!getTransport().getType().equals(TransportType.facebook)) {
                ProbePacket probe = new ProbePacket(this.getJID()+"/"+xmppResource, entry.getUser());
                Log.debug("XMPP: Sending the following probe packet: "+probe.toXML());
                try {
                    conn.sendPacket(probe);
                }
                catch (IllegalStateException e) {
                    Log.debug("XMPP: Not connected while trying to send probe.");
                }
            }
        }
       
        if (getTransport().getType().equals(TransportType.facebook)) {
            Presence p = new Presence(Type.available);
            try {
                conn.sendPacket(p);
            }
            catch (IllegalStateException e) {
                Log.debug("XMPP: Not connected while trying to send presense for facebook.");
            }
        }

        try {
            getTransport().syncLegacyRoster(getJID(), getBuddyManager().getBuddies());
        }
        catch (UserNotFoundException ex) {
            Log.error("XMPP: User not found while syncing legacy roster: ", ex);
        }

        getBuddyManager().activate();
    }

cheer!