This subject seems to have roused some surprisingly strong reactions, possibly because the distinction between advocacy and activism has become blurred for some. Johanna Anderson has covered this subject fully in her blog here, including definitions of the words.
It might be apposite to remember that CILIP has a code of professional practice which "applies ethical principles to the different groups and interests to which CILIP members must relate", and its section D, "Responsibilities to society" begins: "One of the distinguishing features of professions is that their knowledge and skills are at the service of society at large, and do not simply serve the interests of the immediate customer." Of course many people working in the field of librarianship are not members of CILIP and are free not to consider themselves bound by its ethical code. The implication is clear, though: no librarian is an island!
Advocacy can take the form of speaking for the profession outside it, promoting your own service to your immediate users, and promoting your part of it to those who make decisions about your service.
As a subject librarian in a university, I need to connect with staff and students, to promote the collection I'm responsible for and help people to get the best out of it. As a cataloguer, traditionally my role is more hidden than that. For years cataloguers at my place of work have been literally hidden away, first in a basement and now in a different building located at a distance from most of the site libraries. We are much better integrated than we used to be: at one time we were not included in meetings or really in any of the activities other professional staff took part in. Even so I think that much of what we do and how we do it is a mystery to some of our colleagues. Obviously it is important that we communicate better what we do and how it is changing, because in a financially uncertain world if we are not valued we are vulnerable. If we think that what we do is worthwhile, we need to promote ourselves, as we cannot rely on other people agreeing with us. Cataloguing has had an image problem for years - or, as someone once put it to me, "People think librarians are odd but even librarians think cataloguers are odd" (thanks for that!) It's certainly still true - a recent post on LISNPN here ruffled a few feathers ("Is there any position more dreaded ...") and drew this response on the High Visibility Cataloguing blog (set up recently with just this advocacy role in mind). Outside libraries, the role of cataloguer is even more derided - Tim Coates, library campaigner and consultant, reserves his deepest contumely for cataloguers (although I think he excludes academic libraries from that).
As I've worked in special, public and academic libraries, I'm well aware that there's an element within librarianship which looks down on public libraries - I've seen it from both sides of the fence and it has surfaced occasionally during the discussion on advocacy. It is hardly surprising that public libraries are vulnerable to outside attack if they cannot depend on support from within their own profession. Although I work in an academic library now, the years I spent in public libraries were a very important part of my career. I didn't expect it to be like that (I'd listened to too much negative publicity!) but I learned a great deal about librarianship there and it informs everything I do now. (Sweeping generalisation coming up) I think that on the whole those who say they do not use the public library service fall into the category of young/youngish people in full time employment. A word to the wise: being young and in full time work does not last forever!
Advocacy, or at the very least absence of negativity about different spheres of library work, is surely essential. Gone are the days when we can hide away in the stacks and take for granted that everyone will continue to value what we do and fund it. Activism is what happens when advocacy has failed, or when there has been too much complacency (and there have certainly been senior figures in the library profession who have been happy to bend to whatever wind blows for short term advantage without apparently considering the long term for their service). Active involvement in campaigns to save library services is not for everyone, and happily it is not needed everywhere - yet - but one would hope that at least moral support and an understanding of the principles at stake would be forthcoming.