Here is why Labor's share of Income has fallen: Welfare Payments

From Investors.com:

So far in 2013,

• employee compensation is up 11.5% from 2007, 
• proprietors' income is up 3.7%, and 
• income from transfer payments is up 41.5%. 
• Investment income, by contrast, was barely 1% higher this year than in 2007, and that gain was entirely due to rental income. Interest income is down because of the Fed, and dividends have not regained their 2007 level.

In short, the only reason income from work has fallen as a share of personal income in the past six years is not because investors have been collecting a larger share (they haven't), but because the U.S. government has been energetically transferring a much larger share from those who earned it to those who didn't.
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Link: http://news.investors.com/ibd-editorials-viewpoint/111513-679471-entitlement-state-is-to-blame-for-decline-in-labor-income-not-profits.htm?p=full