That's "influential" in quotes because the formula Twitalyzer settles on to to measure such an amorphous characteristic isn't exactly iron clad.
You would have to have joined Twitter yesterday if you believe that the number of followers an account has is entirely indicative of influence. That much Twitalyzer gets right.
But the research firm doesn't really get that much more sophisticated. As the NYT describes it, the secret sauce behind its Influence Index is "the number of times somebody’s Twitter name is mentioned by other users."
That's it!? It's amazing that the cottage industry of companies devoted to Twitter analytics--Klout, Trackur, Twinfluence, to name a few--that have sprung up in recent years have yet to hit upon a compelling strategy to convincingly measure what could be a fascinating metric.
As for O'Brien, Twitalyzer has him well ahead of other TV types including Ryan Seacrest (No. 5), Rainn Wilson (8) and Kim Kardashian (9). Only two individuals have surpassed his influence: Chad Ochocinco (who in all fairness is somewhat of a TV-bred celebrity given his VH1 show, but better known from the girdiron) and at No. 1, Brazilian comedian Rafinha Bastos (never heard of him, either).