Major KOLs such as Sơn Tùng MT-P and many other celebrities are facing this issue. Even Facebook’s “owner” Mark Zuckerberg has only over 9,000 followers, down from more than a hundred million.

There are three opinions regarding this issue. First, it’s a technical glitch from Facebook causing the follower counts to drop, and this will soon be resolved by the platform. Although the displayed follower count on personal pages has decreased, if you check the account information, the full number can still be seen. Next, it could be a move to purge accounts that use tricks to increase fake followers.
Some people spend money to buy “followers,” so this time Facebook is cleaning up these accounts. However, this assessment seems somewhat unreasonable since even accounts like Sơn Tùng MT-P or Mark Zuckerberg are experiencing similar situations.
Another opinion relates to the draft amendment to Decree 72 of 2013 by the government (regarding the management, provision, and use of Internet services and information online) along with Decree 27 of 2018 (on the amendment and supplementation of Decree 72), which is currently under broad consultation by the Ministry of Information and Communications. Accordingly, all social media accounts with 10,000 followers will need to notify the regulatory body, specifically contacting the Ministry of Information and Communications. However, this draft has not yet been officially approved and issued. Therefore, this possibility may be dismissed.
Thus, it is very likely that this is just a temporary technical error on a global scale for Facebook. The technical team of this platform must certainly resolve it quickly.