Since you mentioned page rank, you can also check out graph processing frameworks, eg Pregel/Giraph, which are designed specifically to run graph algorithms on big data.
Replying to this again. Using Scala and Scalding or Spark makes it super simple to do algorithmic stuff. I haven't used Pregel/Giraph ever, so I can't say how it stacks up.
And OP, if you see this, using raw M/R to do things like graph processing is going to be a HUGE pain in comparison to all the libraries built around it.
And OP, if you see this, using raw M/R to do things like graph processing is going to be a HUGE pain in comparison to all the libraries built around it.