Slack most likely has more US customers but - Revenue is not same as users. Slack have tons of free users and some countries also has lower priced plans. - Ma

Slack most likely has more US customers but - Revenue is not same as users. Slack have tons of free users and some countries also has lower priced plans. - Many companies like Amazon etc. probably is counted as US revenue for Slack but they have more than 30% of their employees outside the US. This should not be huge numbers but significant.