We use Optimizely on the Mozilla Developer Network to build and analyze split experiments. We find that the tool helps us move forward with confidence, understanding how changes affect user behavior. We care about accuracy and user experience, so we were concerned when we discovered a problem in a recent experiment that could have affected both.
In Detect if visitors are logged into Twitter, Facebook or Google+, Tom Anthony explains how to determine what social networks your users are logged into. The approach Tom recommends still works today, but the Twitter code needs to be modified to work as expected.