I'd like to talk about the old-fashioned concept of "overstaying your welcome". If you and I chat and exchange business cards, maybe we'll become friends on Facebook. I'm giving you permission to contact me, on either a personal or a professional basis. But not indefinitely.
Seth Godin talks a lot about permission marketing, the idea that a brand like Apple has its customers' permission to send them information about new products. Contrast this with how you'd feel if, say, Toshiba sent you an uninvited email about their new laptop: you'd call it spam. The same principle works face-to-face, too. As co-organizer of a blogging group, member of a Social Media Club, and frequenter of a sort of permanent monthly cocktail party, I meet a lot of people. I even made up personal business cards so I wouldn't have to hand out my company's cards. I've made good friends this way, made peripheral contact with several social networks I otherwise never would have known about, and from time to time I'm asked to support a cause or attend a fundraiser or some such thing. That's all fine.
Here's the thing. If I only met you once, you may not hammer my inbox with weekly inspirational quotes until the next Ice Age. If I've never RSVP'd yes, or even responded in any way shape or form, to one of your event invitations, you should quit sending them.
When does my permission expire? When I can no longer recall what our last conversation was about, when I see your name and remember only your marketing ledes and what you want from me, then I don't want to hear from you. How can you keep that from happening? If we meet in person, talk to me and try to learn what interests me, what I'd like to hear more about, and use it. If we don't meet often, then put me in a Facebook list of people you don't send promotions to until you have, yes you guessed it, had a conversation with me and found out what I want to hear.
It's entirely possible that we have nothing to offer each other, and that's OK. But if you overstay your welcome, don't be surprised if I retract my permission for you to contact me.