IMHO, and it's purely guesswork. But tweets are ranked by Twitter and then Google according to their popularity.
As far as Twitter is concerned a tweets popularity is defined by factors such as, the age of the twitter account the tweet is from, the amount of followers that account has, then how many "retweets" that single tweet had, how many comments and favourites etc it has also. But they could also consider many more things too.
As far as Google is concerned, a tweet is still a web page with its own SEO score. And it will still apply to the Google algorithm. We don't know the exact rules for that algo but we can assume safely that the tweet (like any web page) will be ranked first by age, then by the SEO of the site its on, what links (if any) point back to that page (tweet) how old those links are (if any). That kind of thing.
Also we don't know if Google really does use social media as a ranking factor, social signals, shares, likes, tweets etc. But we can probably assume that it does because sites with lots of social signals usually rank higher because of them. So it (Google) could possibly look at that tweets number of retweets, favourites etc (as well) as they all show on that tweet.
Also, tweets are ranked by the words used within them. That's why lots of Twitter IM's use longtails in their tweets. And they use #hashtags, lots of #hashtags, but not too many. They help as well because anyone searching for those hashtags alone, (whether in Google or on Twitter) your tweet has more chance of being discovered for using it.
I hope that offers you some insight into how tweets are ranked on the big old web!
