Depends on the post. My posts, I read in my voice, other posts in what I think the post-ers are like. E.G Someone who writes somthing like ''OMfg!1!! stfu dude naruto is the best anime evar :3'' vs someone saying somthing like ''Yandere over Tsundere, please post your butthurt to Www.Facebook(dot)com''.
Whatever left a lasting impression on me. Some times it's a cute 20 year old short korean girl's voice or it's semi-british. Other times, it's Yahtzee.