They're not completely free however because users are required to give up a lot of personal information. The bottom line is that there's no monetary cost involved which makes them attractive.
Twitter is attempting to break that mold. The company recently announced the roll-out of Twitter Blue in the US, New Zealand, Australia, and Canada. Twitter Blue is a premium service priced at $2.99 a month which is functionally similar to the Twitter you're currently using with a few key upgrades and enhancements.
Twitter Blue Includes:
Twitter had this to say about the new service:
"We've been listening to and learning from the most passionate and vocal people on Twitter as to what will make their experience more customizable, more friction-less, and simply put -- better. We're invigorated by the feedback we've received so far. The work continues and there's a lot more to build, but in the meantime here's the latest look inside Twitter Blue."
It's a bold idea but it's also a risky move. With so many social media alternatives out there it's unknown how well a Twitter subscription service will be received. One thing that could make it more attractive is a better and more robust moderation system but there will no doubt be at least some users who are sufficiently invested in Twitter to try it out.
In the long run the market will survive. The best outcome for Twitter is of course broad based acceptance of the idea. That will lead to other platforms doing something similar. Worst case is that users will be so put off by the notion that they'll abandon Twitter altogether. That will almost certainly prevent others from even experimenting with paid versions of their services. Time will tell.