It's not tremendously secure from a client-side perspective: if the client machine is compromised, so is your data. But that's not the goal. This is not a tool for spies, this is a tool for everyday people so they can have an unimpaired communication tool while having complete peace of mind that nobody is mining, selling or distributing their data.
What you end up with is a slow, unintelligent website, with fewer features and untargeted advertising that can't recommend friends. It can't do anything useful that social networking butterflies are interested in, and it's one that people who use secure services because it's fun to be paranoid aren't interested in either, because it's not actually secure, and it's a site that's more expensive to run and more complicated.
Don't do it.