The most popular social media platform in Vietnam is Facebook. There are several reasons for its popularity:

1. User Base: Facebook has a large user base in Vietnam, with millions of active users. This makes it a preferred choice for individuals to connect with friends, family, and colleagues.

2. Accessibility: Facebook is easily accessible on various devices, including smartphones and computers. Its user-friendly interface and availability in the Vietnamese language make it convenient for people of all age groups to use.

3. Multifunctionality: Facebook offers a wide range of features and services, such as sharing photos and videos, messaging, creating events, joining groups, and following pages. This multifunctionality makes it a versatile platform for both personal and professional use.

4. Connectivity: Facebook allows users to connect with people from different parts of the world. This connectivity aspect is particularly appealing to Vietnamese individuals living abroad, as it enables them to stay connected with their homeland and maintain relationships.

5. Business Opportunities: Facebook provides a platform for businesses to promote their products and services through pages, groups, and targeted advertising. Many Vietnamese businesses utilize Facebook as a marketing tool, further increasing its popularity among users.

Overall, the combination of a large user base, accessibility, multifunctionality, connectivity, and business opportunities has contributed to Facebook's status as the most popular social media platform in Vietnam.