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"Vendor lock-in" is where users cannot access their data directly and are locked into a particular service. For many years, Facebook employed this method by not allowing you to manage your own personal data, delete it or extract it from the site. External plugins were banned and some developers were sent cease-and-desist letters.
Since, however, Facebook has opened its platform up further to allow users to download their data and to port it to another service, should they wish to.