I'm a jr. marketer w/ experience in social media & influencer marketing. I'm interested in messaging, positioning, product, etc, and think Product Marketing would be a good fit. What are some career tips & suggestions on how to segue into this area?

In your current company, start building a relationship with the PMM team and PMM leadership. Volunteer to take on a few things that leverage your background and help PMM.

One thing that comes to mind is to do an analysis of what is thematically working in social media and influencer marketing, and come to PMM leadership with content recommendations that would have greater impact. "Over the last 2 quarters, the top-performing topic area is x. Customer testimonials / a new demo / a webinar in this area would be really powerful."

Analyze what competitors are doing with social media and influencer marketing, and look at what thematically is working for them. Bring that back to PMM leadership with recommendations for how that PMM leader could outsmart the competition.

Invest in learning the product, and document what worked and didn't work as you embarked on your user journey. Expose your learnings to both Product and PMM leadership. 

Start blogging (personally, or through the company's blog if possible) about the value of the product.