The whole country can be their sample size since they know how we feel based on our emails, posts and browsing history.
Trends and Intensity would be very easy to measure. That’s why they are so into shadow banning. They don’t want our side to know how many people agree with us.
A certain percentage of the population are followers, they want to be on the winning side no matter what. These are the people they are trying to sway.
Having a big sample size does not help unless it is a representitive sample of who will vote in an election. It only makes the the poll more precise in its inaccuracy. As an extreme example to illustrate the difference consider a poll of 100,000 members of a communist fan club vs a poll of only 1000 residence from households in a district who regulraly vote. The “margin of error” of the 100,000 communist fans will be very very small due to a huge sample size. But they are not a representitive sample of voters so the poll results will be garbage.