Marketing vs. Public Relations

Two interesting opposing viewpoint posts over at HubSpot echo some thoughts and questions I've had for a while: Are public relations and marketing two completely different things? Or, do they contribute to one another? Or, are they becoming the same thing?

While many of my teachers and several of those I follow on the web seem to believe marketing and public relations are two separate things, I've come to think of them as integrated with the possibility of becoming one thing under the right circumstances, especially when it comes to social media.

Social media networks like Facebook and Twitter can be used to inform, to interact with publics, and to remind publics of a company's product. For example, at Sierra Repertory Theatre, one marketing person heads the social media efforts, posting links to interesting articles and interacting with customers about theatre, but also linking to ticketing systems, commenting on the success of shows, and reminding customers that they only have a few days left to see shows.

I worked for Sierra Repertory Theatre a little over the summer and found that the theatre company has a one-person marketing department that handles both marketing and public relations. The position basically involved maintaining positive relationships with theatre goers and with theatre reviewers at various area newspapers, but it also involves dealing with subscribers and maintaining subscriptions and donations.

In this case, it would seem that marketing and public relations overlap.

What do you think? Can public relations and marketing work together or become one? Are sales driven by public relations?