One of those daft questions to ponder.

a player takes a direct free kick and the ball cannons into the wall and away to safety.

Do the stats people count it as a shot on or off target and what criteria do they use? Unless they have access to Hawkeye they don't know where the shot would have ended up

The question could equally be posed about someone having a long range shot charged down

Good question... I can't answer it, but it is a good question

I think the definition depends on whether the shot is blocked in the 6 yard box or not. If the block is made in the 6 yard box then the shot is on target (assuming the shot was on target). If the block is made outside the 6 yard box the shot is a blocked shot.
I could be wrong though.

Surely it's only 'on target' if the keeper has to save it. 

I'm sure you could say the same thing about an Own Goal,if a player shoots and its going in and it takes a deflection off the other team the player  gets the goal because it was heading in so that must be on target 

The defender with the bruised wedding tackle could give you an honest answer.

Just emailed opta to ask them, not sure they'll respond though

http://www.optasports.com/about/news/feature-opta's-football-action-definitions.html

Shot on target
Any goal attempt that:
a)    Goes into the net
b)    Would have gone into the net but for being stopped by a goalkeeper’s save
c)    Would have gone into the net but for being stopped by a defender who is the last man.

Shot off target
Any goal attempt where the ball is going wide of the target, misses the goal or hits the woodwork.

Blocked Shot
Any goal attempt heading roughly on target toward goal which is blocked by a defender, where there are other defenders or a goalkeeper behind the blocker.

So it is not a shot on target because the defender is probably not the last man

My guess is it's not counted as a shot, as the wall, or in the other case, the opposing player has done is job in charging the ball down.