As a result of the experiments made possible by the algorithm, Twitter knows more about its users than it ever did before, such as how much they value recency or how they react to seeing multiple tweets in a row from the same person. The company has tried out new features that group tweets about a given topic or hashtag within your feed. It has even experimented with showing you occasional tweets from people you don’t follow, if Twitter’s ranking system shows that you’re likely to want to see them. Twitter can now evaluate the efficacy of such new features by comparing their effects on user behavior to the effects of the ranked timeline and “In case you missed it,” another newish feature. “Our algorithm changes on an almost daily to weekly basis,” Rao said.
Source: Twitter’s timeline algorithm, and its effect on us, explained.