Facebook's newsfeed feature introduced social network users to continuous updates of news about the goings-on in the lives of their friends and contacts. Activity aggregators have turned out to be a pretty useful feature for social networking sites, and can even be a little addictive when done right. Most sites that bill themselves as a social or professional network now have some kind of newsfeed, friend feed, lifestream or other feed.
The Activity module is your best bet if you want to aggregate activities from within your Drupal community. Its API is a little more complicated than similar modules like Activity Stream, but allows for a greater awareness of the context of activity items, so that their appearance can change depending on who is viewing them. Activity comes with seven contrib modules that cover most of the common activities on a community-based Drupal site (creating/editing nodes, adding/removing buddies, voting on content, etc.). This makes it very easy to get activity streams up and running on your site quickly.