The relationship is by default between both, everything that delivers on creating, establishing, enhancing or demolishing an image is a PR skill. I've shared my take on web experts earlier and I will keep shouting about it until the malpractice stops. Feeding social media channels with content is the work of PR, if you hire a proffessional PR firm to do that, it means you are after serious business, let's remember, with social media, it's not awareness we seek ONLY, a successful social media campiagn by PR profs leads to brand engagement, something PR proffessionals have been doing for decades through sponosrship and events.
I really hope I was able to convince the PR lady that PR's role has expanded with the arrival of social media and not changed. Let's go back into history, theater came first, then print, then radio, TV and internet... The more the communication channels are, the greater role PR can and will play, a webexpert who is doing your "social media" now is a TV director, he can produce and impact, but he/she cannot engage without bringing attention to thier program. The PR dudes do that.
I call on all PR proffessionals to support this call by demonstrating real need of PR expertise to secure constant feed of content and messaging versus design and applications/technology.