The getUserId function was removed in the 5.0 release because it was a confusing function. It's been replaced by:
$jfbcLibrary->getMappedFbUserId()
Just update that, and your code should be fine.
One final note, in JFBConnect, we no longer will ask for the publish_stream permission from users, even if you have a "Post to wall on Register" or "Post to wall on Login" message set. You'll need to go into the Facebook API tab of configuration and add "publish_stream" as a requested permission if you want to post on their wall like you're doing above. All past users who have already granted the permission won't be affected. That setting will just make sure new users are properly giving that permission.
The reason for the change was an update to Facebook's policies that messages shouldn't be posted on a user's behalf without them being able to edit and/or approve the message first. If your users are aware of the message, that's fine. If they aren't, you *could* be falling afoul of their terms of service. We haven't heard of any apps being banned for doing such things, but wanted to let you know about that change.
Hope that helps,
Alex