Social Marketing and Dunbar’s Number
Unless the relationship with your company is potentially strong enough to displace that with a cousin, or a work colleague, or a former girlfriend, don’t bother with Social Marketing.

Dunbar’s number is about 150 and it’s the maximum number of active social connections that a person can maintain. Update 07 June 2010: see below for scientific evidence that this applies to Twitter relationships.
Dunbar’s number was first proposed by British anthropologist Robin Dunbar, who theorized that “this limit is a direct function of relative neocortex size, and that this in turn limits group size … the limit imposed by neocortical processing capacity is simply on the number of individuals with whom a stable inter-personal relationship can be maintained.” - Wikipedia (reference)
Friends and relatives certainly count against this number and, I think, relationships with brands do too. This would be all brand relationships: the products you buy, the religion you follow, the company you work for and so on.
Hang on, you say, 150 is nowhere near enough! Actually I think it is, because this is active relationships - ones that affect your day-to-day life. Most of your “relationships” are very temporary and don’t count: former friends that you only think about when you send them a card at Christmas, local politicians that you only remember on voting day, or your insurance company that you forget about until renewal day comes around.
Assuming I’m right, what does this mean for Social Marketing? Basically, you want to maintain active engagement with a client, so that they may think about you when they want to buy, before doing a Google search or asking a friend. Which means you want to be as close to the top of that 150 as possible.
Unfortunately most products and companies are unlikely to be important enough to the people who buy them. A rule-of-thumb is, do you have fanboys?. If your products are not important enough to have at least some obsessive fans, you have an uphill task doing Social Marketing, so it’s probably better to not bother. BTW, I think this is why Social Marketing doesn’t work for most online retailers.
Update 30 April 2011: Frito-Lay is obviously that important (Guinness World Record)!
Update 07 June 2010 - Some hard evidence:
Scientists at Indiana University decided to put Dunbar’s Number to the test by analyzing the Twitter activity of 1.7 million individuals. The team, led by Bruno Goncalves, found that Twitter users’ relationships topped out in the exact range predicted by Dunbar: 100 to 200 maximum. (Full paper: Validation of Dunbar’s number in Twitter conversations.)
See Your Brain’s Twitter Limit: 150 Real Friends -Neurosciencemarketing
Background: I have an MA in psychology from Cambridge, and 20 years marketing experience, so I’m not just speculating wildly. Dunbar’s number is generally accepted, as is an approximate value for it of 150. The use of the word “active” and the extension to brands is my notion - though I doubt it’s original - based on my experience of marketing. - Pete