As an aside, if you never normally credit flights to FB, the problem with taking one DL flight (or flight on any other ST airline for that matter) to keep the miles alive as some suggested in this thread is that, from then onwards, you will only be able to further prevent expiration of these miles through crediting another flight. For some, it might be better to have some other non-flying activity so as to retain the flexibility of being able to further extend the validity of the miles through other means.