If you've ever had an abundance of ads to run on Facebook, but just a few days to run them, then you'll be aware this can be a tricky task to fulfil.
Facebook doesn't rotate its ads evenly, making it difficult to run every ad at the same time and give them all an equal chance of being seen. The Facebook system is designed to select the best ad to show, based on the ad's bid and performance history. So, an ad that performs well will be shown more often than an ad that doesn't perform well. As a result, some of your ads may receive more impressions than others in the same ad set. All this means is that you often have to make a decision between running one banner per day or putting a few ads live together, for a few days.
When one of our clients asked us to run seven promos (in different locations) in a limited space of time, we had just this challenge. We got in touch with Facebook for clarification.
They confirmed that we could run a few ads in one advert set at the same time, but we weren't encouraged to do it. This is because; "Facebook does not currently have a feature to evenly rotate all the ads within an ad set". If you need to run a few ads at the same time, you should look at this as an opportunity to find out what promo/product your target audience responds to best.
Now, there is a new metric that provides insight into why an ad may be delivering in certain way. This is called Relevance Score and it is calculated using the feedback that the ad obtains (both positive and negative).
Although Relevance Score will provide useful information about an ad's performance, you have to keep in mind that bids will also determine the chances of an ad being shown, so the campaign objectives should always be your key metric.