I'm not an expert, but I believe that's why patents and trade secrets exist - to protect your product from simply being cloned. If your product has no secret sauce or patent-worthy mechanism, why shouldn't it be cloned?

the end goal of antitrust rules is (or should be) protecting consumers. if Facebook can kill your startup by adding a couple features to their platform, it implies that users would rather have the feature on Facebook than on a standalone platform, so they are probably not being hurt. if Facebook does the same thing and then charges money for the feature after your startup goes under, maybe the users are getting the short end of the stick.