It seems to me that it proves Twitter's weakness, in the sense that Twitter ultimately must comply with the law, however unconscionable that law may be, but also Twitter's strength --- Twitter did fight the subpoena and may have done so successfully.
It doesn't seem that it proves Twitter's value, except in the sense that WikiLeaks is a savvy enough organization that it wouldn't be using Twitter unless it thought it was valuable enough to be worth the risk.
