Meta has been big enough for antitrust since at least 2010 when they went on an M&A kick[0]. Definitely they should have been blocked from buying Instagram back in 2012. Problem is, by that point governments had effectively hollowed out their antitrust enforcement agencies[1]. So the only option now is to break companies up.

Related note: I don't think anyone should be talking about Threads in the language of competition. Either this displaces Twitter entirely or (more likely) it dies on the vine. While there's been a lot of movement to Mastodon and Bluesky, Twitter is still around. There's no competition between the two; they're serving different markets. The people who jumped ship are the kinds of people who were already getting sick and tired of Twitter's toxicity. The people who remain are either hardcore outrage addicts or journalists and politicians feeding their addiction.

[0] https://en.wikipedia.org/wiki/List_of_mergers_and_acquisitio...

[1] This is often couched in the language of the free market, but practically speaking this was done because bigger platforms are easier to understand and easier to regulate.

Anti trust for a free service?

Unless you mean from an ads perspective, but there are many ad companies.