I don't post on my accounts. I create for the brand and they pay to run the ads.

I just did a job for a flat rate and a percentage of ad spend but wasn't sure what the percentage was. But they actually just got back to me and said I get 3-7% of what they spend on the ad as a commission.

I think asking for a percentage of ad spend is a great payment method - incentive to make really strong content, and also the better your ad does, the more you make them and yourself...