Because posts from online social media, such as Twitter and Facebook, are not yet often fodder for scholarly research, specific reference examples aren't included in the Publication Manual. Well, whenever you need a reference format for something that's not explicitly covered in the manual, you can adapt our examples to meet your needs (see p. 193). I'll show you how, using example posts from President Obama's Facebook and Twitter pages.

To cite a Twitter or Facebook feed as a whole or to discuss it in general, it is sufficient to give the site URL in text, inside parentheses. There is no need for a reference list entry.            

See http://blog.apastyle.org/apastyle/2009/10/how-to-cite-twitter-and-facebook-part-i.html for examples.

She goes on to explain:

As you may have noticed, the Publication Manual does not give specific guidance on how to do this. This is an evolving area, and blog discussions will be considered as we create guidelines related to these new references sources for future APA Style products.
 
What to do in the meantime? Below are examples of one approach to citing tweets and Facebook updates. Until more definitive guidance is available, feel free to use this approach or another that is also clear and gives the reader enough information about the source to be able to locate it.

First, here are screenshots of my examples from Twitter and Facebook (click to enlarge):
 
   

The suggested reference list entries below generally follow the format for citation of online sources (see pp. 214-215):

BarackObama. (2009a, July 15). Launched American Graduation Initiative  to help additional 5 mill. Americans graduate college by 2020:  http://bit.ly/gcTX7 [Twitter post]. Retrieved from  http://twitter.com/BarackObama/status/2651151366  

Barack Obama. (2009b, October 9). Humbled.  http://my.barackobama.com/page/community/post/obamaforamerica/gGM45m [Facebook update]. Retrieved from http://www.facebook.com/posted.php?  id=6815841748&share_id=154954250775&comments=1#s154954250775  

Here's the rationale I used for presenting each element in the reference:

• I included the author name as written (not changing BarackObama or Barack Obama to Obama, B.; see Example 76, p. 215). For simplicity's sake and to ensure accuracy, it's best to include names as written for Facebook and Twitter citations.
• Alphabetize under B, not O, ignoring the space (i.e., BarackObama and Barack Obama are treated the same, so you would next arrange them chronologically; see also section 6.25 of the manual, pp. 181-182). 
• The date includes the year and day, but not the time. The date gives ample specificity without adding an element of how to format times, which isn't done anywhere else in APA Style. 
• To differentiate among posts from the same person in the same year (or even the same day), you can include "a" or "b" after the year, in chronological order. If you have only one post from the writer in a year, then it is not necessary to include "a" or "b." 
• I included the whole post in the title position (mine included a URL, so I included that too). Facebook updates, however, might be quite long-if that is the case, you might use a truncated version of the post in the title position. 
• It's helpful to provide a description of form inside brackets, such as Twitter post or Facebook update. 
• The URL leads directly to the post rather than to the feed in general, in order to be as direct and specific as possible about what is being cited. Click the date and time stamp beneath the post in question (seen in the screenshots) and you will be taken to the individual status update page with its own URL.

For in-text citations, parenthetical citation may be easiest:

President Obama announced the launch of the American Graduation Initiative (BarackObama, 2009a). He also stated that he was "humbled" to have received the Nobel Peace Prize (Barack Obama, 2009b).  

One last issue is retrievability. Because online social media are more about live updates than archiving, we don't know if these status update pages will still be here in a year, or 5, or 20 years. So if you are writing for publication, it may be prudent to self-archive any social media updates you include in your articles (check out this post by Gunther Eysenbach on some ways to do this).

See the complete article at: http://blog.apastyle.org/apastyle/2009/10/how-to-cite-twitter-and-facebook-part-ii.html