Yeah I understand that there's a difference between the sampled population and the actual population of interest, but you can't discount the results of on account of that unless you can meaningfully show a non-zero covariance between the response variable and likelihood of non-response.
By all means it's a caveat but it doesn't make these results entirely non-informative.
In any case I cited iid conditions to explain why asking all their friends is certain to produce a useless estimate of the population proportion.