Bid vs cost on Facebook refers to the different pricing models used by advertisers to run their ads on the platform. The bid is the maximum amount an advertiser is willing to pay for a specific action, such as a click, impression or conversion, while the cost is the actual amount charged by Facebook for that action. The bidding process is dynamic, meaning that the cost of an ad placement can change depending on the competition for the same audience. Advertisers can set their bids manually or use automatic bidding, which lets Facebook optimize the bid based on the campaign's goal and the available budget. Understanding the bid vs cost on Facebook is crucial for advertisers to create effective campaigns that reach their target audience and achieve their desired outcomes while staying within their budget.