I answer on AskHistorians, but don't post there. But I'd love to see activity there too! Mostly I run three history meme communities (HistoryMemes, RoughRomanMemes, and ShermanPosting), and four communities for sharing historical images of interest (HistoryPorn [not my choice of name, it's just for photos], HistoryIllustrations, HistoryRuins, and HistoryArtifacts)