Facebook Inc. knows, in acute detail, that its platforms are riddled with flaws that cause harm, often in ways only the company fully understands. That is the central finding of a Wall Street Journal series, based on a review of internal Facebook documents, including research reports, online employee discussions and drafts of presentations to senior management.

Time and again, the documents show, Facebook’s researchers have identified the platform’s ill effects. Time and again, despite congressional hearings, its own pledges and numerous media exposés, the company didn’t fix them. The documents offer perhaps the clearest picture thus far of how broadly Facebook’s problems are known inside the company, up to the chief executive himself.