In the wake of the last recession, one industry seemed to rebound faster than most: businesses that sell products and services to other businesses. B2B companies saw their sales soar in 2011 as soon as other businesses decided to start reinvesting in their own operations. 

But more recent stats show that while the industry is still healthy, it's slowing down.

With $15.8 billion in combined sales, the B2B sector ranks third on Inc.'s list of the biggest industries of 2015. It’s bested only by IT services and health care, which boast annual revenues of $17.5 billion and $24.4 billion, respectively.

To be fair, third isn't a bad place for the industry to be. And indeed, many of the 486 companies within the category are thriving. The Roswell, Georgia-based business social network, for instance, is the 10th-fastest-growing company on the Inc. 5000. Last year, the company, which was founded by CEO Bill Wade, saw its revenue tick up 18,888 percent since 2011, to more than $31 million.

But as is clear in the chart below, after revenue peaked at $66.7 billion in 2011, the B2B sector as a whole saw sales sink--precipitously--in each subsequent year. And that's as U.S. economic growth has moved north--the economy’s second quarter performance ticked up a solid 2.3 percent.

So what happened?

One possible explanation for the slowdown is a retrenchment in business spending--or, more likely, a return to normalcy. After companies were able to satisfy the pent-up demand post-recession, their investment levels are settling on a more stable course.

In the July reading of the National Federation of Independent Business's Small Business Economic Trends survey out Tuesday, the number of business owners planning capital outlays in the next three to six months rose 1 point to 24 percent. The advocacy group indicates that while improved, the reading isn't altogether promising. "Investment plans remain historically sub-par, and owners have little interest in borrowing to support investment spending that promises little return," writes NFIB chief economist Bill Dunkelberg in the report.

Want to learn more? Check out the chart below to read between the lines of the Inc. 5000’s top industries.


Biggest Industries: Total Revenue by Industry

Below, view the industry aggregates of Inc. 5000 companies' total revenue for the last five years (from 2011 - 2015). Some industries banked more cash than others.