Not everyone is OK with their posts being shared by bots (maintained by SE) randomly on social media.
I would add that posting on social media platform means giving some rights on the content to the platform, which often doesn't respect the Stack Exchange license (viz., CC BY-SA 2.5/3.0/4.0).
For example, see this answer by user6726 on Are tweets an intellectual property? regarding Twitter terms of services:
Under the Twitter TOS, [...] you do license the content:
By submitting, posting or displaying Content on or through the
Services, you grant us a worldwide, non-exclusive, royalty-free
license (with the right to sublicense) to use, copy, reproduce,
process, adapt, modify, publish, transmit, display and distribute such
Content in any and all media or distribution methods (now known or
later developed). This license authorizes us to make your Content
available to the rest of the world and to let others do the same. You
agree that this license includes the right for Twitter to provide,
promote, and improve the Services and to make Content submitted to or
through the Services available to other companies, organizations or
individuals for the syndication, broadcast, distribution, promotion or
publication of such Content on other media and services, subject to
our terms and conditions for such Content use. Such additional uses by
Twitter, or other companies, organizations or individuals, may be made
with no compensation paid to you with respect to the Content that you
submit, post, transmit or otherwise make available through the
The totality of conditions, including the Twitter Rules, is
ever-evolving and not apparently contained in a single link. From what
I can tell, there is no condition that prohibits a user from copying
tweets into a book. There are numerous statements about "respecting
copyright" which refer to taking material that is not licensed to
Twitter and redistributing: nothing about redistributing licensed
material.A plain reading of the first bold sentence says that you can
make your content available to the world, not restricted to
Twitter is also notorious for selling at a high price APIs to access the tweets, so a Stack Exchange user may disagree with giving their work to Twitter for free.
With that being said, Stack Exchange never showed any interest in preventing bots from masscopying Stack Exchange questions to Quora, so I doubt they'd care much about the social platform licenses.