And then Pregel is believed to be used for calculating PageRank. How do it work? When the new version is added to the "webtable" while crawling web pages periodically, each processor of Pregel scans latest version of anchors in its partition, and updates the graph structure. The several vertices received message from newly-created or updated vertex will be reactivated and begun to recompute PageRank incrementally.
Although Graph module (Pregel clone) of Apache Hama supports dynamic graph operations, I never tried yet. I'll post more details if it works fine.