Twitter has 450 million monthly active users at a generous guess.
Facebook's MAU: 2.98 billion
Instagram's MAU: 1 billion
Twitter, is an (un)surprisingly small fry when it comes to social networks.
Unlike most, Facebook actually knows how to build a social network, and grow it.
Not really considering they desperately tried to get out of buying it.
Further, Twitter has laid off 75% of the people who could help combat this push.
You are describing Twitter from 2-3 years ago. It is not is such a strong position now, and people are actively looking for something else that provides similar functionality. By bootstrapping from an existing social graph, it's a really strong possibility this is Twitter's death blow.
Threads is not a threat at the moment. Everyone I follow has already gone back to Twitter and hasn’t posted on threads since the first day.
As for relevance... A few more great business decisions from Musk, and Twitter's relevance will plummet to unrecoverable depths. All Facebook has to do with Threads is not do obvious mistakes.
But that all is completely beside the point. The original claim was "They are going head-to-head with one of the largest social networks".
"Large networks" are counted in terms of MAU and hours of engagement. And Twitter is rapidly losing it's relevance. Because FB's (and everyone's) largest competitors are
1. Time. There's only 24 hours in the day
2. Social networks and products grabbing that time: TikTok and video streaming services
Twitter doesn't really enter the equation.