The report, created by Hopper HQ, an Instagram scheduling tool, surveyed more than 2,000 UK social media marketers.
Of the respondents, 60 per cent said they were "forced" to work at least two unpaid hours on weekends, meaning they would be owed an average of more than £2,400 per year if they were eligible for overtime, the report says.
One respondent said they worked up to 20 unpaid hours per week, leading Hopper HQ co-founder Mike Bandar to say unpaid overtime appeared to have become "a necessary evil" within the industry.
Bandar said: "It's astonishing to see just how much time marketers are taking out of their weekday evenings and weekends to work... [and] it's a testament to how far marketing agencies and their teams go to meet client needs."
Elsewhere in the survey, 46 per cent said they did not see Twitter as a "useful" social media network.
When asked how much their client or employer spends on Twitter ads each month, 72 per cent said £0. This is in comparison to an average of £12,000 spent on Facebook, and £7,000 on Instagram.