go here:
bitcoinmonitor.com
Do you see those dubious tx that draw those funny tails all over the graph, one after another? Also look up into the sky: Can you spot those 5200.xx BTC tx acurring all the time?
Those are nothing but bitcoin launder tx sending bitcoins back and forth in between hundreds (if not thousands) of alibi addresses, to make tracing impossible.
And before anyone gets any ideas... that technique by itself is completely useless. Unless you have different coins from unrelated sources, no amount of shuffling bits around is going to do you any good.
It's like trying to follow someone carrying a package on a huge expanse of sandy desert. They can wander back and forth over their track as much as they want, but all someone else has to do is look at the edges of your tracks, notice that no-one else has joined, and be confident that the tracks exiting the mess were the same as the tracks entering it.
What blockchain.info's "mixing" service does is sends in a helicopter to grab the package from the first guy wandering the desert, flies a couple kilometers, and then gives it to someone totally different to continue the journey.
The more likely answer as to what's going on is actually really simple: some shared wallet service has a fixed limit of 5200BTC that it's willing to store in a single address, yet at the same time is designed to bring all the coins it has into a single address. The repeated transactions are just a known side-effect of the way Bitcoin is implemented; you can't spend part of a transaction. That means if you have an input transaction of 5200BTC to an address, to spend even 1BTC of that requires a transaction spending the whole thing. Bitcoin monitor's website just doesn't know if the 5199BTC or 1BTC is the change.
Said transaction pattern corresponds to what sites like bitlaundry do, ensuring that every coin going into the service is mixed together, so that any coin leaving the service could have come from any input.
Going back to my tracks example it's like a hundred package couriers entering the desert, huddling together and shuffling the packages between each other, than leaving again. Anyone trying to follow tracks would never be able to figure out which package came into the whole mess from what courier.
