I have tried using both Hadoop (55 node cluster at Cornell) and a single AWS High Memory double extra large instance with 32GB memory.

I have found that since the Twitters social graph is small enough to fit in the memory, a single instance with huge amount of RAM is much more efficient, especially when your algorithm iterates over nodes in the network.

You can read about it here:

Hadoop based results: www.akshaybhat.com/LPMR/

Results using a single High Memory instance AWS instance www.akshaybhat.com/LPMR/GRAPHLAB

Even the startup hunch has taken a similar approach and use a single machine with large amount of memory rather than a hadoop cluster.