The FAANGs had soft agreements to work together on labor by not hiring each other's workers. That seems to fit the definition of a cartel:

"group of independent market participants who collude with each other in order to improve their profits and dominate the market"

I suppose we might argue that even the FAANGs together aren't large enough to constitute the full market. Other companies compete over the workforce of FAANG-eligible workers. The long tail of such SV employers with lower employee headcount a is still probably large in the aggregate. Though even apart from no-poaching agreements I've seen it speculated that FAANGs may employ large #s of talented people somewhat unnecessarily purely to keep them out of the hands of current competitors or potential startups that could grow into competition. That's a bit too conspiratorial for me though, at least without more evidence