Archiving social networking sites

Archive-it (http://www.archive-it.org/) is an interesting archiving service by the Internet Archives, which is "able to capture most publicly available web pages, including social networking sites" (https://webarchive.jira.com/wiki/display/ARIH/Archiving+Social+Networking+Sites+with+Archive-It). However, I continue to wonder how robust this tool/service can be to assist individuals to collect and preserve their own, private digital content produced in the social networking sites.