That’s one summary of a recent study from Pew that compared Twitter reactions to public surveys when measuring opinion on the same political events. Alex Fitzpatrick, noting widespread efforts to use Twitter as “an accurate reflection of public opinion,” is more detailed with the findings:
According to Pew, in some instances — Barack Obama’s reelection, the first presidential debate and a federal court ruling on California’s same-sex marriage ban — the reaction on Twitter was “more pro-Democratic or liberal than the balance of public opinion.” However, other events — Obama’s second inaugural speech, John Kerry’s nomination as Secretary of State and Obama’s 2012 State of the Union address — elicited a more conservative response on Twitter than they did in opinion polls.
Pew also pointed to the general negativity of political tweets, which perhaps suggests people are more likely to tweet about something about which they disapprove rather than vice-versa. That would affect Twitter sentiment data vis-a-vis public polling data, as Twitter data is comprised of opinions from people who weren’t directly prompted to share an opinion whereas public opinion polls rely on respondents’ answers to a series of questions.