We investigate the conditions in which cooperation is dominant in social media using the models of evolutionary games of public goods games with diminishing marginal utilities and attempt to clarify the mechanisms of prosperity of social media and effect of diminishing marginal utilities. Cooperation dominant situation corresponds to that in which social media thrives, i.e., posting articles and taking reactions by comment lead to more benefits result than do nothing as free-riders. A number of studies examined what mechanism contributes to maintain cooperation dominant situation by using evolutionary game, but their models don't include the effect of diminishing marginal utility. Thus, we developed an abstract model of social media using utility function with diminishing marginal utility and we experimentally investigated the effect of diminishing marginal utilities in the meta-rewards and SNS-norms games. The result showed that the games with diminishing marginal utility could model real SNS activities more suitably than those without it.