I would be more interested to know, whether OP is talking about reviews on WHT or in general social media/websites ? Like the ones who promote their affiliate links saying EIG sites are the best in the business.
its not necessary that fake reviews are only shared by new members, it can be done by old members too. In the past I have seen in online marketplaces where buyer is seeking accounts which are over a year old, has a decent post count too – to either buy account or to pay someone for sharing their product/content.
As far as contribution is concerned, not everyone joins a site to contribute – they may be looking for something specific, once they are done – they may not visit the site for god knows how long & can share a review thanking the provider for their services. Not everyone registers on the forum either, I have no clue how long I was a guest myself before joining wht.
Just because a review is by a new member doesn’t mean its fake & just because its shared by an old member it’s true.