Your posts appear to be being deleted because your account does not fulfil the subreddit's karma/age standards. As a means of fighting spam, certain subreddits will impose minimum karma or account age limits. These specifications aren't the same everywhere.

If one of your postings vanishes off your friend's Wall, it's most likely because you or the profile owner erased it — either intentionally or accidentally. Facebook will remove a post that breaches the site's standards in a minority of circumstances.