The inbound marketing industry, still emerging, is already playing a vital role in how marketers are doing their jobs, although challenges remain, according to the just-released fifth annual State of Inbound Marketing Survey from HubSpot.

"Inbound marketing not only has staying power, it's growing. It has proven itself to be much more than the next shiny technology or the latest buzzword," Meghan Lockwood, author of the 1164-page report, writes in its introduction.

"The research shows that nearly 60% of marketers have adopted inbound marketing strategies and more than 80% of those executing inbound marketing have integrated it into broader company goals. People are also investing more in inbound, as budgets for this strategy have grown nearly 50% in each of the past three years."

What follows are five compelling insights from the survey, including challenges confronting inbound marketers.

Insight: Inbound marketing is still hard to define

Although 60% of marketers surveyed say they have adopted inbound strategies, 19% are unsure if the nature of their marketing activities qualifies as inbound.

Challenge: How do you budget for and measure something you can't define?

Insight: Marketing and Sales are still divided (or maybe they just need to talk more)

73% of companies with a defined Marketing and Sales agreement (agreed-upon definitions and processes) are using inbound activities, which shows some connection between implementation of inbound marketing and effective sales.

Challenge: How do you prove the bottom-line contribution Marketing makes to the company if you can't connect lead-gen efforts to sales?

Insight: Inbound is a relatively new industry demonstrating major growth

Nearly 50% of marketers say they increased their inbound budgets for 2013.

Moreover, marketers are allocating 34% of their overall budgets to inbound activities, or 11 percentage points more than they are spending on outbound activities.