Suppose I'm a private pilot and see a post in a closed group on Facebook where someone is looking for a scenic flight around my area. If I offer them a flight with me, sharing costs equally, am I holding out? Does it matter if I know the person vs. if I don't? Thanks!
This is almost certainly holding out: someone asks for a flight and you agree to provide it. You weren't going to make the flight until the person asked, and you're receiving compensation for it. This is from the Bobertz interpretation:
Absent a bona fide common purpose for their travel, reimbursement for the pro rata share of operating expenses constitutes compensation and the flights would be considered a commercial operation for which a part 119 certificate is required.
The Haberkorn interpretation that acpilot mentioned is somewhat similar to your scenario in that it talks about using Facebook but with the difference that the pilot advertises for passengers, rather than passengers asking for pilots. You can read the whole thing and although the FAA said they can only comment on specific cases, not general examples, they did make clear that they're concerned about situations where:
there is an interest in carrying passengers with whom there is no previous personal relationship and that the offer to accept passengers is being made to the general public
But even if you already know the individual, it's probably still a commercial operation.