A lot has been speculated already on how Google ranks content out of the realtime web (s.a. Google announces Realtime Search Features). Now an official statement has been reported at the SearchEngineWatch Blog.
The evaluation of realtime content (here Twitter) and by this the ranking happens by similar principles like the ranking of common content. Parameters factored in are the reputation of followers and not alone the quantity of followers, which is analogous to relevant links by well established and relevant websites. Read more at the following article.