  With the recent advance of micro-blogs and social networks, people can view
and post comments on the websites in a very convenient way. However, it is also
a big concern that the malicious users keep polluting the cyber environment by
scamming, spamming or repeatedly advertising. So far the most common way to
detect and report malicious comments is based on voluntary reviewing from
honest users. To encourage contribution, very often some non-monetary credits
will be given to an honest user who validly reports a malicious comment. In
this note we argue that such credit-based incentive mechanisms should fail in
most cases: if reporting a malicious comment receives diminishing revenue, then
in the long term no rational honest user will participate in comment reviewing.
