One question though: what, if anything, does ifttt do to detect/prevent infinite loops? If I create a task to copy new photos added to flickr to instragram, and another task to copy new photos added to instagram to flickr, what happens?
Caveats to the above are that you'd need to make sure that accounts in the channels are tied to single users (or join together the graphs of multiple users where they share channel accounts). You'd also need to handle odd edge cases with trigger fields and addins - such as a user @mentioning themselves and triggering an action to reply to the user who mentioned them.
I have to admit, I look at stuff like this and my first reaction is "hmm, what could possibly go wrong... could we somehow use this to calculate prime numbers perhaps?"