BEST PRACTICES

FACEBOOK:

- Using Twitter hashtags in a Facebook post, and/or auto-posting from Twitter to Facebook is a no-no. 

Why?
Due to Twitter's unpopularity with most of the public, which can be blamed on numerous factors, most notably that it is not very user friendly or intuitive, causes many people to have negative feelings towards Twitter, Twitter users, and the Twitter language (#s, RTs, FFs, etc.).  By using Twitter language in the non-Twitter space you are essentially bringing in a stranger to a comfortable and intimate gathering of Facebook.  You can ultimately alienate followers when you use this unknown Twitter language, doing the opposite of what you want to do on Facebook, which is create a sense of shared space and communication. At this time Twitter has 1/8 the number users that Facebook has (or about 100 Million active users total). When looking at total users, the chances of ticking off your Facebook population is high, and the time saved not creating separate posts minimal.

Addendum:

Although it is frowned on to post from Twitter to Facebook, the opposite does not apply. It is okay to have your Facebook post automatically tweeted. Since there is not a special "Facebook language" or other symbols associated exclusively with Facebook, the chance of ticking off a Twitter user is low. Plus, the utter hegemony of Facebook in the Social Media sphere leaves little expectation that any web-space will be Facebook-free (for better or worse).