In that case, tracking a re-joining user with email makes more sense than mobile phone, since emails cannot be transferred. As far as my thinking goes, as soon as I delete my account, Facebook's ability to track me with that mobile number should go. There is no point in digging my phone number from the networked people's data and try to connect with me again. For me, it reconfirms my point that to quit facebook, we need to sacrifice all online persona related to it.