I retract the "libertarian website" part *. A reformulated version of my remark is: The respondents who identified themselves as socialists wouldn't probably agree with Heilbroner's definition of socialism. I was addressing futility of arguing over definitions rather than a possible libertarian bias.

*) My mistake was caused by the title Library of Economics and Liberty and the fact that the site is maintained by the Liberty Fund, whose description of themselves sounds typically libertarian.