Abstract—This study investigates the factors impacting the frequency of slang usage on social media, with a particular focus on the phrase "flex" on Facebook in Vietnam. We used a mixed-approaches strategy, which combined quantitative and qualitative research methods. Data were gathered from a representative sample of 1000 Vietnamese Facebook users chosen from a population of almost 1,000 using a systematic sampling procedure with a sampling interval of k=5. Surveys and interviews were done to collect detailed information about user demographics, societal influence, and personal sentiments toward slang usage. The data was analyzed quantitatively using SPSS. The findings show that demographics, social influence, and personal opinions have a considerable impact on the frequency of slang usage. Younger users and those with more social media activity are more likely to utilize slang phrases such as "flex." The study also found that slang usage is frequently motivated by a desire for social identity and approval. These findings help us understand how language evolves in digital communication and how social media influences linguistic trends.