Facebook’s latest error shakes advertisers’ confidence

Facebook Inc. is offering millions of dollars in credits to some advertisers after discovering a glitch in a tool that tells advertisers how effective their ads may be in driving results, such as getting consumers to download an app or purchase a product.

Facebook’s “conversion lift” tool overestimated some campaign results for 12 months, the company quietly told its advertisers this month. The glitch skewed data that advertisers use to decide how much money to spend with the company.

It isn’t the first problem Facebook has discovered in its systems to measure advertisers’ campaigns, and it is not likely to dent Facebook’s ad revenue.