It is recognised that Monad Shoal is the best place in the world to see thresher shark which gather for cleaning there. These shark are pelagic - not territorial and have likely come here for thousands of years for cleaning.
Thresher are deep water shark and they are very 'skittish' is disturbed. They are light-sensitive (large eyes) and upset by movement and flash photography.
The more popular dive shops do take ever increasing numbers of people diving and some in the group do not follow the instructions set by the dive operator. We have seen this many times.
If you want to see threshers, then dive with an operator who has strict policies and exhibits good control over their divers. However, the rule of averages states that the more divers, then the more chance you have a 'rogue' diver who can spoil the dive for you. We have witnessed dive operators who allow their divers to travel off the drop off and over the wall where the threshers ascend to be cleaned. Rarely do these divers see threshers.
Tip; select a dive operator which limits the number of divers and has a guide who understands the behaviour of the Thresher shark. Dive Link are one of the best here for this. We have made 14 days diving with them and saw threshers on 90% of our dives. On other days, we saw devil rays, manta and shark.
All our diving was done away from areas where groups of divers from other companies were milling around.
Onto the 'rock' - this is Gato Island. Gato Island is one of the best sites in SE Asia for macro life and has a healthy shark population. Diving the tunnel in the afternoon in small numbers allows you to see white tip shark in a blue water cavern; diving in higher numbers will virtually eliminate your chance of seeing this.
Re negative comments, I do not know the number of dives/experience that people have before going to Malapascua, but choosing to dive with an operator that offers good comfort, convenience and PR can and does attract less experienced divers. This is not an issue if the operator is well organised.
Just understand that threshers are reclusive and very sensitive creatures that come close when divers understand how to behave and respect the rules imposed by the dive operator.
Just one renegade diver can ruin the entire dive of all divers kneeling on the shoal for 30 minutes to get one sighting of the world's most beautiful shark.
Finally, nothing is guaranteed in life - Monad is the best place for Thresher but nobody can control the shark. They are wild animals. 2 days diving is not enought there IMO. 3-5 days is about right for Monad. If you are a serious photographer, then 10 days is better.