Algorithms for finding dense subgraphs are an important subroutine in many data mining tasks, in particular, community and event detection in social networks. Huge amount of data is produced daily by Facebook and Twitter users, calling for efficient algorithms being able to handle large amount of data evolving over time. In this talk, we shall present algorithms for efficiently finding dense subgraphs in an evolving graph and discuss some preliminary results on how they can be used for automatically finding interesting events in social networks.