If a guy doesnt respond back to your message on Facebook but reads it does it mean he isn't interested?

Well, there are a lot of possibilities.

1) As you said, he may not be interested

2) He might feel like he would annoy you by replying

3) He just might not have the time at the moment

4) embarrassed that he is talking to a girl?? hahaa

5) He might just not know what to say back

So many possibilities.

You can't really tell what a guy is thinking, since there brain waves are not completely accurate :) hope I have helped