John Carson said:
"I think if the campaign goes well, and everyone involved is a hero, then people don’t care about disclosure.
But, if the sh*t hits the fan, then people search for accountability and look back — ruefully — with hindsight about the disclosure issue.
By then, of course, it’s too late!"
That really sums it up for me. This emphasizes the importance of disclosure from the beginning. While, of course, everyone plans their PR strategies to go well but that is not how it always happens. If something goes wrong, which Murphy's Law teaches us that it always will, no disclosure means a much bigger mess than if you just tell the truth (i.e. not lie through omitting) from the beginning.
That said, as another commenter pointed out, giving disclosure on social media like Twitter where you have a limited space can be difficult. Paid tweets are becoming a big deal and the FTC is stepping in. One of the solutions that many PR practitioners are coming up with is some sort of indicator in the tweet that lets the reader know it's a paid or sponsored tweet. Stowe Boyd of microsyntax.com suggests putting "AD" at the beginning of the tweet. Brian Carter of TweetROI takes it a step further and makes the distinction between a paid tweet and a sponsored tweet. Paid tweets, where the company dictates the content, would be indicated by "AD," while a sponsored tweet, where the company does not decide the content, would be indicated by "SP." There are even more elaborate idea such as color-coding and different fonts for different types of tweets. No matter which way will eventually win out, the main goal of all of these ideas is to provide disclosure and keep Twitter-marketing ethical.