Awkward issue. This stackexchange is dedicated to question about open data and all things open source. It strikes me then that the very platform it has been created on is not open source. It is true, the devs behind StackExchange have suggested they may consider opening the source sometime in the future. But still... is this not in conflict with the principles of BlueObelisk? A purist might say it is even hypocritical.
What's more, there are free and open alternatives budding on the internet, why not support their initiative and use them instead?
Two threads from meta on the topic:
Two clones (like I already said, they are budding :D):