Maura, as a marketing lead in B2C, I appreciate how you are positioning your agency. I typically look for similar scale when I evaluate the right outside PR resource. That way, as a client, I can ensure I will get an appropriate share-of-mind.
PR has changed insofar as the tools that can be used to build your brand awareness have changed so I look for professionals who are familiar with all these new techniques and have shown a proficiency in creating successful strategies that incorporate innovative approaches that are on-brand. That would presume they have done their homework to understand your brand and your industry.
As for social media, I haven't found there is a lot of consistency with PR agencies that are also equally successful managing social media. I usually like to see that there is a separate business unit or division (even if it's one or two people) focusing on social media design, strategy and community management.
Hope that helps.