Bad data is not the same as no data. For example, the fact that somebody is willing to manipulate a statistic is information in itself, and could be correlated

Bad data is not the same as no data. For example, the fact that somebody is willing to manipulate a statistic is information in itself, and could be correlated (positively or negatively) to an outcome. Part of working with big data is analysing all your signals coming in, and being able to determine which are useful and which aren't. Social media may be a noisy, biased signal, but I doubt it is so noisy, or so biased, that no useful information can be obtained from it.