[junto]

If I was the NSA I'd force/put some piece of network hardware that mirrored all VPN traffic exiting PIA's endpoints. I would assume that the US, UK and DE endpoints might be monitored without PIA's knowledge (unless they own the data centre and/or upstream provider?).

Then it is fairly simple to start pattern matching the unencrypted traffic exiting your endpoints by matching HTTP headers for each client. Then all they would need is for a VPN user to acces a website that leaks the user's identity and you can back match their previous traffic.

For example, you search for information on "how to make a bomb" via the VPN. Your browser sends the the HTTP headers, Accept-Language set to Accept-Language: ar-YE,en-US,fr-FR,de-DE;q=0.5 and a user agent of Mozilla/5.0 (Windows NT 6.2; Win64; x64; rv:21.0.0) Gecko/20121011 Firefox/21.0.0. Those HTTP headers aren't unique, but they vastly narrow the search scope.

Now as that user you visit your Facebook page, and those same matching HTTP headers are passed. Boom, you've just leaked your true identity.