A disadvantage of using social media for marketing research is

the sample of individuals from whom content is gleaned may not be statistically representative of the marketplace.

Explanation:

The product that is promoted by Marketing team tend to be suitable only for costumers with specific set of characteristics. (Could be depended on their age, income, gender, etc)

The data that is gained from internet more often than not will come from anonymous Account. You can never be sure about the account's owner identity and characteristic.

Which mean that the person from which the data is taken do not necessarily represent the market demographic that the marketing team want to target.