When you delete your tweets, then yes, assuming people retweeted using the Retweet button, those retweets are also deleted. Replies are not deleted because tho

When you delete your tweets, then yes, assuming people retweeted using the Retweet button, those retweets are also deleted. Replies are not deleted because those belong to the senders, not you. As for Facebook, it is true that deleting your account does not delete your messages. That's something they have been criticised for. However, I believe deleting your messages does result in recipients being unable to access them (except from cache, of course).