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Other social networks paint a similar picture, as you can see in the image below, with one notable exception: Facebook. 52 percent of Facebook’s UK audience account for 98 percent of the time spent on the site.
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This is mostly due to slightly different habits of Facebook users when compared with users of other social networks. The average time a typical user spends there is higher, and therefore (Nielsen claims) you'd get similar results if you defined heavy users as those who spend over 5 hours on Facebook each month. Interestingly enough, a huge percentage (9%) of Facebook users spend over 20 hours on the service every month.