Pinterest stock was sliding in Friday’s premarket as a narrow third-quarter earnings beat failed to win over investors who are fretting about the image-sharing platform’s mounting artificial intelligence costs. Futures tracking the benchmark index and contracts tied to the gauge of small-cap stocks were both down 0.2%. Pinterest’s adjusted third-quarter earnings of 40 cents a share beat the 34 cents a share figure analysts were forecasting, according to FactSet consensus estimates, while its sales of $898.4 million just about topped expectations.