The FTC actually says explicitly that starting a Twitter post with #ad is OK [0]. They don't mention Instagram in that document. I imagine the legally can't be overly prescriptive due to first amendment issues.

FTC sent notices out to people using "creative" tags on Instagram like #sp or #partner. They did not send notices to people using #ad. I would take that as a sign that they think #ad is reasonable for now.

> What about a platform like Twitter? How can I make a disclosure when my message is limited to 140 characters?

"The FTC isn’t mandating the specific wording of disclosures. However, the same general principle – that people get the information they need to evaluate sponsored statements – applies across the board, regardless of the advertising medium. The words “Sponsored” and “Promotion” use only 9 characters. “Paid ad” only uses 7 characters. Starting a tweet with “Ad:” or “#ad” – which takes only 3 characters – would likely be effective."

[0] - https://www.ftc.gov/tips-advice/business-center/guidance/ftc...