There are several alternatives to Twitter available, such as Bluesky and Mastodon, but these have struggled to gain traction.
Threads has a significant advantage because it is connected to Instagram, and the hundreds of millions of users already on that platform.
How does Threads work?
On Threads, posts can be shared to Instagram and vice versa and can include links, photos, and videos of up to five minutes in length.
However, some early users on Wednesday reported problems when uploading images, hinting at teething problems.
Users see a feed of posts, which Meta calls “threads”, from people they follow as well as recommended content.
They are able to control who can “mention” them and filter out replies to posts that contain specific words.
Unfollowing, blocking, restricting or reporting other profiles is also possible, and any accounts users block on Instagram are automatically blocked on Threads.
While Meta stresses ties to Instagram, media coverage has focused on its similarity to Twitter, with some investors describing the app as a “Twitter killer”.
On Saturday, Twitter boss Elon Musk restricted the number of tweets users could see on his platform per day, citing extreme “data scraping”.
It was Mr Musk’s latest push to get users to sign up to Twitter Blue, the platform’s subscription service.
Twitter has also announced that its popular user dashboard TweetDeck will go behind a paywall in 30 days’ time.
Since Mr Musk took over, many users of Twitter have publicly expressed their dissatisfaction with the platform and his stewardship – citing erratic behaviour and political views.
Last month, Mr Musk and Meta boss Mark Zuckerberg agreed – possibly in jest – to a cage fight, and Mr Zuckerberg’s early posts on Threads mentioned his interest in mixed martial arts.
While Threads will be available in the UK, it is not yet available in the EU because of regulatory uncertainty, particularly around the EU’s Digital Markets Act.
But the company says it is looking into launching in the EU.
That act lays down rules on how large companies such as Meta can share data between platforms that they own. The sharing of data between Threads and Instagram is part of the issue.
Meta maintains protecting privacy is fundamental to its business.