A few days ago, the Florida Bar’s Board of Governors adopted Ethics Opinion 14-1 (which had been circulated earlier this summer) on whether an attorney may advise a client to “clean up” social media pages (Facebook, Twitter, Instagram etc.) in anticipation of litigation to “remove embarrassing information that the lawyer believes is not material to the litigation matter.” You can read the full opinion, which is very short, here.
The opinion mentions opinions from other jurisdictions on the same question and concludes "that a lawyer may advise the client pre-litigation to remove information from a social media page, regardless of its relevance to a reasonably foreseeable proceeding, as long as the removal does not violate any substantive law regarding preservation and/or spoliation of evidence." However, if the information is removed from the social media page, "the social media information or data must be preserved if the information or data is known by the inquirer or reasonably should be known by the inquirer to be relevant to the reasonably foreseeable proceeding"
In the end, what the opinion seems to be saying is that there is a difference between "removing" the evidence from the social media page and "destroying" the evidence. Removing it is simply hiding it, but not destroying it. Also, given the added requirement to preserve the evidence after removal, presumably the evidence will continue to be discoverable.
In other words, what the opinion seems to say is that the client can take something that was public and hide it, but has to preserve it in case the other party in litigation asks for it in discovery, at which point the client may have to produce it.
It seems to me this is not different than the rules that would apply to any other kind of evidence.