Twitter is not a social network. Not primarily, anyway. It’s better described as a social media platform, with the emphasis on “media platform.” And media platforms should not be judged by the same metrics as social networks.
Social networks connect people with one another. Those connections tend to be reciprocal. Facebook even checks in on you now and then to make sure you’ve actually met the folks who are sending you friend requests. As a social network, its chief function is to help friends, family, and acquaintances keep in touch.
Media platforms, by contrast, connect publishers with their public. Those connections tend not to be reciprocal.