For decades, companies such as Equifax have acquired employee work histories and compensation data from employers to help lenders, landlords, hiring managers and other customers perform background checks of individuals. But these big databases are vulnerable to theft and error, and workers are sometimes surprised their records are included, according to privacy activists. Equifax said it follows all laws and welcomes additional voices in the industry. In the letter to the FTC, San Mateo, California-based startup Certree said that Equifax and Experian are providing financial incentives like a slice of their revenue to employers to gain exclusive access to payroll data. Equifax also has deals with payroll software vendors that help employers process paychecks. The letter describes the agreements as anticompetitive and potentially unlawful.