It looks as though Facebook's stock isn't the only thing that's dropping, as a new report suggests that its userbase is also decreasing, albeit only slightly. According to comScore, as reported by Reuters, the number of unique visitors coming to the social network in the US was 158.01 million in May, a drop from the 158.69 million who visited in April and 158.93 million in March. However, while a smaller number of people are visiting the site, those that do are doing so for longer — in May users spent an average of 380.8 minutes on the social network, up from 378.9 minutes in April. According to Reuters, comScore has changed how it calculates unique users, making it difficult to see how Facebook is faring compared to last year — using its older method comScore calculated that 157.22 million people visited the social network in the US last May.