There's a new social networking service that makes aggregating by topic easier. It's called Storify:
Storify is a way to tell stories using social media such as Tweets, photos and videos. You search multiple social networks from one place, and then drag individual elements into your story. You can re-order the elements and also add text to give context to your readers.
You can request an invite, or you can simply create an account using your Twitter Login. Then you click and drag stories from other places on the web into one single thread, similar to a blog post (but much easier). Here's a demo: