I work in Info Sec. There's a big difference between data stored locally on a device and messages on a SM platform. Facebook for example doesn't use end to end encryption so assuming a warrant can be obtained and the messages weren't deleted by both parties some time ago (no one really knows how long), then they'll be there. For apps like Signal that use end to end encryption the messages aren't stored anywhere but the 2 or more communicating devices and so the company cannot provide them even if compelled.I think your supposition is correct.
For example....
Following the San Bernardino terror attack, federal law enforcement really wanted to access the perpetrators I phone- with out triggering the auto erasure of the information.
The case was a national level terror case and the FBI, CIA, NSC etc. have enormous resources in regards to data re-construction etc.
But...not even they were confident that they could reconstruct the erased data on the phone. So, they took Apple through a long court battle to unlock the phones w/o erasure. Eventually, the FBI withdrew the Court request because a third party helped them out regarding the erasure.
Anyways, using the erased I phone data fear of the FBI, one could extrapolate that once some social media exchanges erase- they truly erase to the extent that not even national level resources can readily reconstruct them.
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Apple Fights Court Order to Unlock San Bernardino Shooter's iPhone - Nouvelles de sécurité - Trend Micro FR
The US federal court has ordered Apple to comply with an FBI request to unlock the iPhone of one of the San Bernardino shooters. Here’s why Apple said "no," and why it matters.www.trendmicro.com
As an aside, if you want to see the extent of the data the US government sweeps up on its citizens look no further than the X-Keyscore NSA program which Snowden revealed, the kind of thing that when China does it, people are horrified by it. Browsing history, chats, emails.. The whole works.