I'm glad you asked about KPIs. As Product Marketers, we don't have the luxury of a single metric or even a couple metrics. We own the health of the story & vision our company is selling. I say health intentionally. It's not just that we own the story (we do) but we also need to make sure it's landing amongst our key segments, that we have the right segments, our sellers can actually deliver the story (if you're B2B) and on and on. And it's something that we need to be monitoring regularly --- which is where KPIs come in. I see the cornerstone KPIs in four categories: Interest, Velocity, Win Rate, Cross-Sell.
For the B2B context, these are some specific examples I would expect to see in these categories. For B2C - these can still apply, but win rate will be more around engagement or purchase conversion.
Interest - Opportunity Growth - how many new opportunities is the business creating each month? Your goal is to grow this number! If it starts to stagnate - go figure out why. Is your content stale? Did your target segment change? Is a competitor stealing attention?
Velocity - Deal Progression - how long (days/weeks) does it for deals to move through the sales cycle? Your goal is to see if you can shorten it - through sales plays, content, customer stories, ROI calculators, etc. Look for where deals are getting stuck and stay close to your sellers.
Win Rate - Win Rate :) - what percentage of deals become closed-won revenue for the business? Your goal here is, no surprise, maintain or improve your win rate! Also included here is a win rate against top competitors, but as an input into the overall metric. If you see your win rate dipping that's where you need to quickly diagnose what's behind it.
Cross-sell - Cross-sell :) - what percentage of accounts purchase additional products after becoming a customer? Your goal here is understand if your story continues to work post-sale. Is the vision your company sells compelling customers to expand their business with you? Are youe expansion plays working as well as your land plays?
The challenge we face as a product marketer is we don't control a single part of the process - rather we influence all parts of the buyer journey from first touch all the way to cross-selling. Looking at one metric will never give us the full picture. Use metrics like those above to measure the impact of the work you are doing and look for opportunities to make improvements.