Getting the last post of all threads
Hard to give full answer without understanding the ReaderFeedback table definition.  I will point to some areas of concern.


A LIKE clause that has a wildcard at its beginning will never be able to use a typical B-TREE index on the field you are comparing against.  This will always require a full table scan.  For this reason, you probably will need to look at using full text search with appropriate full text index.
You are using sub-selects, which oftentimes introduce performance problems. Can this be designed away?
It appears you are implementing a parent-child tree hierarchy, which typically is a little painful to work with in MySQL (or most any other relational database).  You might want to check out this article: http://mikehillyer.com/articles/managing-hierarchical-data-in-mysql/  on managing hierarchical data in MySQL.  You might for example find better read performance from using the nested sets model shown there (though nested sets are typically more complex when it comes to inserting/updating nodes).