Technically, it is very expensive to track activity on other services on the order such a system easier to program within your service.
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Well, very questionable usefulness: if the user is interested he will activity to show if the user is not interested, it is pointless to reward his activity, he still would not be interested as soon as the payments end there will be outflow of such users, in General, a "buy" attention and activity is to deceive themselves, users not nagradami on the tools go. Better to keep the service interesting in itself.