I think some categories of public post on FB are linkable and searchable, but I'm not sure exactly what, and I suspect comments on linkable-searchable-things are not themselves linkable and searchable, and (see the next point) I have no reason to think that what's linkable and searchable now will remain so in the future.

AFAIK you're right about "searchable", but the timestamp of any comment on Facebook is a permalink. For example, here's a link to the earliest comment on EY's latest Facebook post.