Under one scenario, the shy become more extroverted and everyone enjoys their new bon mots and witty asides. Gains from trade increase all around. Under another scenario, shyness and extroversion are part of a larger positional game. Some people take the anti-shyness drug, but the previous extroverts, facing new competition for sex and friends, become even more extroverted, thus feeling more strain. Many of them start taking the drug to stay ahead. The previously shy exhibit more "juice," so to speak, but without much net result in terms of an improved life since they are still coming in second, so to speak. And those who don’t take the anti-shyness drug are even worse off than before, given the new and higher standard for extroversion.
Some of the remaining shy, however, might in fact feel relieved. If the new standard of extroversion rises so high that they can’t possibly meet it, they might, to some extent, withdraw from social competition. The truly shy might even form social clubs and band together in the interests of promoting shyness. If they can signal that they do not take the drug, their shyness might become more socially acceptable than before.