Social media provided a valuable platform for suicidal individuals to post their thoughts and behaviors. However, existing suicide studies using social media data have failed to recognize users' heterogeneity and the temporal nature of suicide risk. To identify social media users at a high risk of suicide, we examined the variations in the trajectories of post volumes among users on the r/SuicideWatch subreddit to (a) investigate the heterogenous patterns of change in suicide risk and (b) characterize their linguistic features. We collected and analyzed post data every half year from March 2019 to September 2022 among users on the r/SuicideWatch subreddit (N = 6,163). Using the growth-based trajectory model, we identified two distinct trajectories of post volume among r/SuicideWatch subreddit users. A small proportion of users (12.08%) were labeled as at a high risk of suicide, with a sharp and lasting increase in post volume during the pandemic, while the majority of users (87.92%) were categorized as being at a low risk of suicide, with a consistently low and mild increase in post volume during the pandemic. Particularly, the high-risk group was distinct in using words related to anger, sadness, feelings, health, motion, and death during the end of the collected period. Based on the two trajectories of post volume, this study has divided users on the r/SuicideWatch subreddit into suicide high- and low-risk groups. Our findings indicate heterogeneous patterns of change in suicide risk among r/SuicideWatch users, which demonstrated distinct linguistic features. We recommend conducting real-time surveillance of suicide risk using social media data to provide timely support to individuals at a potentially high risk of suicide.