In social media sampling, there are many issues. Two of them are: 1) the silent majority problem and 2) the grouping problem.
The former refers to the imbalance between participants and spectators: can we trust that the vocal few represent the views of all?
The latter means that people of similar opinions tend to flock together, meaning that looking at one online community or even social media platform we can get a biased understanding of the whole population.
Solving these problems is hard, and requires understanding of the online communities, their polarity, sociology and psychology driving the participation, and the functional principles of the algorithms that determine visibility and participation in the platforms.
Prior knowledge on the online communities can be used as a basis for stratified sampling that can be a partial remedy.