By Katie Paul and Jaspreet SinghJuly 29 (Reuters) - Meta Platforms reported a 91% drop in second-quarter free cash flow on Wednesday, underscoring the financial strain of the social media giant's costly AI buildout amid an uncertain payoff. The Facebook and Instagram parent company reported free cash flow of $784 million in the second quarter ended June 30, down from $8.55 ‌billion a year earlier, sending its shares down 10% in extended trading. Meta's free cash flow was the lowest since late 2022, when the company was facing similar investor scrutiny over spending on its ambitious metaverse bet. The company reported second-quarter earnings per share of $6.18, missing analysts' average estimate of $7.22, according to data compiled by LSEG. "Meta isn't spending billions on AI infrastructure just to make Facebook and Instagram better.