We conduct a large-scale study of online community variation in language. We show that factors of efficient communication, which have been shown to drive crosslinguistic variation in lexical semantic systems, also play a role in within-language variation across 1926 English-language Reddit communities. We study variation in stancetaking behaviour, a domain where efficient communication may be influenced by social motivations for language use. We find that communities indeed have efficient stancetaking systems, particularly with respect to their own communicative needs. However, contrasting with crosslinguistic work, we find that communities are often not optimized for their needs. Moreover, we find that community-level social factors correlate with how optimized they are. These results highlight the importance of accounting for social pressures for language use when studying how efficient communication drives variation.