Facebook’s US unique monthly visitors dipped in May to 158.01m, indicating that the social network’s growth is flattening in more mature markets. The stats from comScore are down marginally on the 158.69m US unique visitors recorded in April. Nevertheless, critics could argue that the dip, however small, is concerning news for Facebook as it indicates a lack of returning users amid claims from some of ‘social media fatigue’. Despite the fall, the average time spent on Facebook during May was up month over month to 380.8 minutes, from 378.9 minutes in April.