Twitter is anonymous, but 100% public. There is no way to limit your audience to a smaller group.
FB is not anonymous and mostly public. Private groups do exist, but you still mostly need to have an actual name on your account.
TikTok & Snapchat are mostly for people who show their faces, not anonymous.
WhatsApp shows everyone everyone's phone numbers if they are in the same group, not anonymous. You can also be forcibly added to a group of hundreds of people you don't know without permission.
Telegram and Discord are more like old-school IRC was. Just a nickname and that's it, no personal info needed. (IRC did have the hostname, but some networks masked it)