The Red Cross has many major marketing initiatives. At the end of the day - their marketing efforts are executed to make non-customers become customers; in this instance the Red Cross is typically soliciting donations from customers. They may also try to get existing customers to make another donation. They run advertisements, events, promotions, and publicize their activities to make a conversion. The success of their marketing campaigns is determined by the success of their fundraising initiatives, attendance at major events, and number of people they are able to help after a disaster.
The Red Cross also manages advocacy campaigns and advocates separate from their marketing campaigns. If you look at their "About Us" page you'll see all of the different causes the organization supports. Even if you had never heard about this organization - it is easy to determine if the Red Cross is something you would want to financially or verbally support. If their funding was in jeopardy it would be easy for the Red Cross to show why it is important to continue funding certain projects. They have concrete numbers available regarding the number of people they have helped, areas in which they have served, and organizations with which they have partnered. You don't necessarily have to be a customer (or Donor) of the Red Cross in order to understand their mission and support their cause - and support funding their efforts.
Stepping back into Library-Land - a marketing campaign may be initiated to increase use of e-Resources on campus, promote a new or renovated space, or improve customer service on campus. These are all targeted measures that improve or increase a product or service on campus. It is about customer conversion; creating new customers or increasing repeat customers should (usually) be the end goal of a library marketing campaign. A library advocate would be a person that would stand up in front of the Provost and argue that the library funding should not be cut. A library advocacy campaign may call people to action to support library funding, space, or services on campus.
Successful marketing will likely create library advocates - the more a student or faculty member uses your services the more likely they are to support you in the long run. However, the campaigns have different goals and measures of success. A marketing campaign should increase transactions (reference stats, visits, instruction sessions - whichever measure you're using) but an advocacy campaign should increase support (be it financial or emotional) for the library. It's easy to see why the terms are frequently used interchangeably - but the subtle differences can actually add up very quickly! It's important to consider what your goal will be when you're writing a strategic, marketing, or advocacy plan. Are you trying to increase funding? Are you trying to add a new service? Are you trying to improve user experience? The actual goal of your campaign will determine what type of plan you create, how it is executed, and what will be considered successful.