Correct, but developers are directly responsible for the performance and stability of the platform. I highly doubt the designers specified in their designs that

Correct, but developers are directly responsible for the performance and stability of the platform. I highly doubt the designers specified in their designs that the website should constantly display: "Sorry, we couldn't load posts for this page" as mentioned in the OP. If we are talking about poor interaction design patterns and unintuitive conceptual models with no prior usability testing/UX research then designers/researchers deserve the blame (assuming devs didn't botch the implementation and create usability problems), but this isn't the issue at hand... platform stability is and that's the job of the developers who code it.