Phrase to describe the "act of sharing an opinion"
Throwing your two cents in.
Is an idiomatic way of saying, sharing an opinion.
I think the word you are looking for is opine.
From Oxford Learners:
to express a particular opinion
To share an opinion is to spread the word, i.e. to leave it where it wasn't before.
When social media makes your opinion run among "similar people", then you're not spreading the word, not really sharing your opinion. Beware of it in your research.