Get used to tweets from people you don’t follow in your Twitter timeline. It’s an official feature.

Twitter recently began adding tweets to your timeline that have been favorited by people you follow. The decision has been a controversial one, but it looks like it’s here to stay. Twitter has now formally changed its definition of your home timeline to note that it will add in content that it thinks you’ll want to see.

Previously, Twitter had this definition on it’s “What’s a Twitter timeline?” page:

What’s in your home timeline:

When you sign in to Twitter, you’ll land on your home timeline.

• Your home timeline displays a stream of Tweets from accounts you have chosen to follow on Twitter.
• The newest updates are at the top. You can reply, Retweet, or favorite a Tweet from within the timeline.
• Clicking anywhere on a Tweet in your timeline expands the Tweet, so you can see photos, videos, and other information related to that Tweet.

Note: You may see content from accounts you do not follow, such as promoted Tweets, Retweets from accounts you follow, or content that may be relevant to you. Read more about promoted Tweets here, and Retweets here.

A recent update to the page has added this paragraph, which expounds upon the above note:

• Additionally, when we identify a Tweet, an account to follow, or other content that’s popular or relevant, we may add it to your timeline. This means you will sometimes see Tweets from accounts you don’t follow. We select each Tweet using a variety of signals, including how popular it is and how people in your network are interacting with it. Our goal is to make your home timeline even more relevant and interesting.

Since the wording has been added to an official help page, it’s safe to say that this is no longer an experiment. For purists, this means that the service no longer lets you customize your feed exactly to your liking, instead moving toward a Facebook-like mix of opt-in and recommended content.

Of course, if showing faved tweets from people you follow turns out to be too unpopular, Twitter can always adjust its algorithm. Still, a line has clearly been crossed: Twitter is betting its future on its data curation, rather than our manual curation.

Twitter deserves our begrudging respect for its willingness to rethink the most basic building blocks of its service: tweets and timelines. Over the past year, Twitter has reordered your timeline with a new conversation UI and added images to a text-only medium. In the long-term, expanding the definition of your timeline is what’s best for Twitter as a mainstream platform, but doing so will upset hardcore users along the way.

Welcome to the new, new, new Twitter.

Image credit: Matthew Keys / Flickr