Social media is a dominant communication environment for university students, yet public discourse often portrays platform shaped language as devalued and harmful to language ability. Evidence for these deficit claims is mixed, and links between social media language practices and self-perceived first language proficiency remain underexplored. Using a sequential explanatory mixed methods design, this study examined how social media language devaluation, defined as engagement with and endorsement of four linguistic feature dimensions that include repetition and exaggeration, form simplification, semantic generalization, and multimodal dependence, relates to Chinese college students’ self-perceived language proficiency in social and learning contexts. In the quantitative phase, 502 students completed a survey with a 16-item devaluation scale (α=.774) and a 19-item proficiency scale (α=.888). Spearman correlations showed a significant positive association between devaluation and overall perceived proficiency (ρ=.237, p<.001), strongest for social situation proficiency (ρ=.378, p<.001) and weak for learning situation proficiency (ρ=.102, p =.022). Regression results indicated that devaluation explained 7.5% of variance in overall proficiency and 17.1% in social situation proficiency, with subgroup differences by gender and major. In the qualitative phase, interviews with 20 participants suggested these features support stance taking, affiliation, and reduced social anxiety, alongside clear awareness of register boundaries between informal online interaction and formal academic contexts.