This is an awesome question and one that I've thought a ton about as we've scaled. I touched on this a bit in another answer (“When thinking about adding new talent to your team - how do you structure focus areas?”) but will reiterate and expand here.

As our company has scaled, I don’t think it’s necessarily the case that our key stakeholders as a Product Marketing team have changed, per se—rather, I think of it more along the lines that our key partners in supporting those stakeholders have evolved. Meaning: on a macro level our key stakeholders still remain:

Product management: supporting the commercialization of our product strategy through integrated go-to-market efforts
Sales: empowering our Sales Org (which includes both Acquisition and Customer Success) to communicate the value of our solutions in a concrete, differentiated manner
Marketing: educating our Marketing Org on the core value props, providing insight into upcoming product launches and just generally keeping them aprised of all things relating to our go-to-market efforts

On a more tactical level, it becomes necessary as the organization grows to develop key partnerships with other teams in order to effectively support these stakeholders. This is partially true simply because Product Marketing traditionally does not scale in headcount at the same rate that the rest of the Product and Sales Orgs do. For example, we’ve gone from 2 product managers and a few product designers when I started at Sprout Social nearly 6 years ago to having nearly 20 PMs and 20 product designers supporting a much bigger product portfolio today. Even more impressive, we’ve scaled a 50 person Sales team which was almost entirely inbound when I started to a proper inbound/outsound Sales Org with well over 300 people and comprised of Acquisition, Customer Success, Growth and Onboarding.

In order to continue to provide the level of support and service we've developed key partnerships with the following teams:

Solutions Engineers have become an incredible partner in technical know-how and truly understanding the needs of the buyer. We partner closely with our SE team to execute betas, maintain our demo environments, better understand pain points and gaps in the product, and to facilitate product feedback mechanisms to our Product Team. This has been one of the most valuable parnterships for us.
Sales Enablement has helped us scale our sales training efforts by taking on much of the tactical execution and providing great insight into the varied needs of each sales team—something that was incredibly time intensive for us prior to them coming onboard.
Product Ops is a newer discipline for our company but one that has been invaluable in bridging the communucation and partnership gap between Product and the rest of the company. This is something that Product Marketing historically took on in the absence of a dedicated Product Ops person/team. This is more of a nascent discipline. If you want to learn more about Product Ops, Kim Blight is a great person to connect with!