Great write up Jamie. Some interesting things to think about and lots of useful examples.
I suspect that for most personal websites the idea of fair use will give people enough protection for reply contexts. Of course it will depend on their jurisdiction as fair use can vary by country or potentially even within countries in terms of how it is applied.
I would almost have to think that barring particular legislation and precedent that people/companies who are explicitly providing Open Graph Protocol or similar meta data on their websites are explicitly granting a license to use that content as the only use for that data on most systems is to provide it for creating contexts on services like Facebook, Twitter, etc. Facebook likely created OGP as a proprietary format to give itself broad legal protection for just such use cases, though I suspect they parse pages and take titles or other snippets when OGP doesn’t exist. Naturally some large systems like WordPress may push OGP into code without the site’s owners being aware of what they’re potentially giving away, so the area is really murky at best. It would be beneficial to consult an attorney to see what their best advice might be or if there are precedents with respect to these areas.
For future reference, here is the relevant section for Fair Use from Title 17 of the Copyright Law of the United States:
107. Limitations on exclusive rights: Fair use40
Notwithstanding the provisions of sections 106 and 106A, the fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include—
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work.
The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors.