Just to bring this question up to date and back into the fold, here is some updated data on our site's answer rate. The Stack Exchange Data Explorer has a wealth of information that can be extracted with queries such as this one:
Is the rate of answered questions declining?
The rate of questions with answers has slipped from 90% to 77% since Jan of 2013:
date answered total ratio
-----------------------------------
5/1/2011 2459 2515 98%
6/1/2011 2836 2913 97%
7/1/2011 3203 3292 97%
8/1/2011 3542 3659 97%
9/1/2011 3893 4033 97%
10/1/2011 4213 4376 96%
11/1/2011 4564 4767 96%
12/1/2011 4899 5130 95%
1/1/2012 5173 5434 95%
2/1/2012 5538 5843 95%
3/1/2012 5995 6339 95%
4/1/2012 6479 6884 94%
5/1/2012 7062 7554 93%
6/1/2012 7725 8294 93%
7/1/2012 8355 9022 93%
8/1/2012 8999 9775 92%
9/1/2012 9604 10480 92%
10/1/2012 10176 11157 91%
11/1/2012 10918 12017 91%
12/1/2012 11710 12958 90%
1/1/2013 12354 13722 90%
2/1/2013 13137 14686 89%
3/1/2013 13819 15551 89%
4/1/2013 14585 16623 88%
5/1/2013 15317 17664 87%
6/1/2013 16032 18720 86%
7/1/2013 16675 19685 85%
8/1/2013 17364 20779 84%
9/1/2013 18057 21844 83%
10/1/2013 18757 22907 82%
11/1/2013 19525 24140 81%
12/1/2013 20189 25247 80%
1/1/2014 20758 26230 79%
2/1/2014 21518 27477 78%
3/1/2014 22151 28744 77%
If you compare this with other mature sites (by switching the site selector at the bottom of the query page) you'll see a similar trend. This obviously does not answer why this is happening, but I would assume it's a sign of maturity and probably closely follows the site's user population.
I found another very telling query here:
Number of users answering or questioning.
Year Quarter Questioning Answering
---- ------- ----------- ---------
2009 3 1 1
2010 1 2 2
2010 2 2 8
2010 3 215 253
2010 4 231 226
2011 1 382 330
2011 2 497 396
2011 3 533 404
2011 4 554 424
2012 1 796 550
2012 2 1209 670
2012 3 1259 772
2012 4 1475 822
2013 1 1743 974
2013 2 1918 1007
2013 3 1962 995
2013 4 2058 1033
2014 1 1693 819
This seems to confirm something that I've suspected has been going on, which is that there are a whole lot more people asking questions than answering questions than there used to be.
This is probably a natural progression with site maturity. When a site is young it is filled with enthusiastic, knowledgeable users who have been around since the dawn of (the site's) time and are both knowledgeable with the site (they know, for example, that answers yield twice the reputation that questions do) and the subject matter. As time goes on and word gets out about the site being a great resource to ask questions, the demographics change and you get a whole lot more people looking for information rather than offering it.