This is an interesting idea, but I'm not sure it really works.
For one thing, we are talking about Americans. Why? Is it simply because they are the the only people we have this data on? Are others in the world seeing similar levels of economic dissatisfaction? Do these levels of economic pessimism corrolate to amount of social media use? If Americans are especially pessimistic, why would social media use have a bigger impact on the than others?
I think that increased social media use is definitely a factor in increased unhappiness (he said on social media). But my default hypothesis here is that when you ask people "how do you think the economy is doing?", the question they answer is "do you feel nervous about your financial situation?" And even if the economy is doing well, a lot of Americans have a lot of legitimate concerns - the cost of housing, the cost of healthcare, the cost of transportation, the cost of college. Debt they've taken on. Office jobs being offshored or (supposedly) taken by AI. Tariffs, and a government that is trashing its relationships with its best trading partners.