The idea behind effective networking is to build contacts with which you can share opportunities. Sharing is the key concept here. If you create a network of contacts hoping that they will think of you for every opportunity, although you never share opportunity with them, you are mistaken.
There is a synergy and general feeling of community that will lend a genuine empathy to your dealings. Make no mistake; a me-only perspective will impact the effectiveness of your professional networking. This is clearly most effective as a two-way street.
Also, when people begin to view you as a person who knows people and can make those connections vital to professional development, they will contact you. This has the added benefit of keeping you in the communication cycle. You will be more aware of what types of opportunities are being developed, furthering your value as a professional contact.
I used to tell my employees that I wanted our company contacted if a client needed a window washer or his carpets cleaned. It wasn't because we provided that service, but because I wanted to be viewed as the company that provided solutions and knew others in different lines of work, with the same attitude. We would facilitate relationships, increasing our value.
I cannot count the number of times I've been contacted by someone in my contact list to find assistance for projects outside of my purview. They simply believe that I will know someone. And when I do not, I scan my list of contacts to find someone I believe will know the right contact.
In the end, I am typically able to assist the person who contacted me.