You are assuming that what was interesting for the initial users is exactly the same that is interesting for the increasing mass of joining users, but as you in

You are assuming that what was interesting for the initial users is exactly the same that is interesting for the increasing mass of joining users, but as you increase the number of users, things that have mass appeal have more likes, retweets, etc. So an interesting but more niche post will potentially receive less average attention than before. Therefore, a niche community loses its defining qualities as the number of users increase. For advertisement purposes, total engagement triumphs, so this is perfectly fine and lucrative for the platform itself, but the quality is not necessarily maintained.