Data: FactSet; Chart: Matt Phillips/AxiosAs AI expenses bite, Meta's second-quarter free cash flow fell to its lowest since just before the social media giant launched its layoff-laden "year of efficiency," the company reported Wednesday.The big picture: The AI boom is transforming major tech giants from asset-light, cash-spewing behemoths into indebted owners of a sprawling fleet of super-expensive data centers. And nobody knows whether those data centers will eventually turn out to be profitable investments. The intrigue: That rapidly changing financial reality is why the bond markets are starting to show signs of concern about the size of the bets these companies — once thought to be virtually invulnerable — are making. Zoom in: Free cash flow provides one of the "cleanest" snapshots of how a company is actually doing, as measured by the real dollars that end up in its coffers.By the numbers: The cost of Meta's massive AI investment binge is beginning to bite. It reported $30.12 billion in purchases of property and equipment, plus another $962 million for lease payments.Combined, that ate deeply into the $31.86 billion the company's operations produced during the three months ending on June 30.As a result, free cash flow was just $784 million during the quarter — the lowest since the company could only cough up $173 million in free cash in the third quarter of 2022, when Mark Zuckerberg & Co. was still focused on the Reality Labs division. Remember the Metaverse?Meta's shares fell sharply in after-hours trading Wednesday, following the release of its Q2 report. Meta isn't alone. Google's parent Alphabet also reported that its free cash flow actually fell into negative territory for the first time on record in its most recently reported quarter. Yes, but: Not all tech giants are the same. Microsoft posted strong quarterly results and showed free cash flow of around $20 billion during its most recent quarter, also reported Wednesday. Its shares jumped after hours.