I'm not entirely sure you answered the question. We understand corporate responsibility vs. Individual responsibility. The question wasn't about the division of that responsibility (and ultimately I disagree with your conclusion on this. Personal responsibility is limited by knowledge and personal agency, and those of lower education or inclinations towards addictive tendencies may find it excessively difficult to curb social media addiction).

The question was about how you justified marketing a harmful product.

Perhaps you believed your actions themselves did not contribute to the harmful aspects of the product or that your responsibility (inability to change the negative aspects or influence on the cause) are significantly low.

In either event, I don't believe this was really answered.