People will grow tired of seeing your book promotion posts and will soon stop noticing them.
That’s why you need to come up with new ways of trying to sell your book beyond the standard “out now” posts. Running giveaways is a good step. People love freebies and get invested in the chance that they might win. And when they don’t, they might now feel that they now want your book enough to buy it.
As well as this, competitions are a good way to get your audiences engaging with your promotions. This means they will likely perform better.