[Old Pedant]

Try this...

SELECT p.postId, p.roomId, p.uniqueId, p.title, p.post, p.image, p.createdOn, r.created
FROM posts p
INNER JOIN (
   SELECT u.postid, MAX(u.createdOn) AS created
   FROM (
     SELECT p.postId, p.createdOn
     FROM posts p
     WHERE p.deleted = 0
         UNION 
     SELECT p.postId, c.createdOn 
     FROM comments c
     LEFT JOIN posts p
     ON p.postId = c.postId
     WHERE p.deleted = 0
     AND c.deleted = 0
         UNION
     SELECT p.postId, r.createdOn
     FROM replies r
     LEFT JOIN comments c
     ON c.commentId = r.commentId
     LEFT JOIN posts p
     ON p.postId = c.postId
     WHERE p.deleted = 0
     AND c.deleted = 0
     AND r.deleted = 0
   ) AS u
   GROUP BY u.postid
) AS r
ON p.postId = r.postId
WHERE p.deleted = 0
ORDER BY r.created DESC

I think that's right. Note how using MAX( ) ensures that, indeed, you will get only one "created" date/time for each postid.

If I have the sense of it wrong, you might want to change MAX( ) to MIN( ).