It's quite common to see pagerank change after the google PANDA update. It is quite likely the PR with come back within a few weeks.
My toolbar and tools show as PR 9. There is no way Twitter.com can be PR NA. It is either because there is some bug in the tool that you are using or your IP is blacklisted by Google query for using it extensively!
Twitter is still PR9. You are most likely just using a worthless website to check it out. Even if twitter would drop in PR it would never become a N/A because that's only for website that has not been ranked yet.