In such a case, investigators whose services we use have their methods, and they are familiar with evidentiary requirements. For them, comments on social networks are just that—comments. Those specialists are skilled enough to determine what evidence is relevant in a harassment investigation.
The impact of social media goes beyond that context alone. When allegations against a public figure, such as an MP, are made, it is a matter of the individual's reputation. However, during the investigation, those aspects are not necessarily taken into account.