Our experience on the BCDG has been similar. Less than 1% of the membership provides 90%+ of the content and building code related discussions and pertinent news. 90% of the members are the "lurkers" (they are reading the Q&A, but not participating by providing questions or answers). About 9% are the intermittent users.
We are very fortunate to have a large membership that allows us to provide continuous and relevant content by an awesome group that represents less than 1% of the total membership.
Social media for small organizations: Why size matters
The 90-9-1 principle popularized by Jakob Nielsen says that if you have 100 visitors to your online community site or social media presence, your visitors' participation will look like this:
As Jake McKee puts it:
90% of users are the “audience”, or lurkers. The people tend to read or observe, but don’t actively contribute.
9% of users are “editors”, sometimes modifying content or adding to an existing thread, but rarely create content from scratch.
1% of users are “creators”, driving large amounts of the social group’s activity. More often than not, these people are driving a vast percentage of the site’s new content, threads, and activity.
And that's just your users. Most people never visit your site at all; some people visit your site once or twice a year. So among 2,000 members, you'd be lucky to have 200 who visit on a monthly basis; 1% of that 200 is just 2 people you can count on to contribute to your site on a regular basis.
That's why sites need a large potential audience to be able to build a solid, self-reinforcing mass of user-generated content. Figure that to attract regular, repeat visitors you need a minimum of 2-3 content contributions per day, and to create a sense of a user-owned, user-driven community, you need at least 10 contributions a day (successful sites have dozens or even hundreds).
To get to 10 contributions a day, flip that pyramid upside-down. Let's err on the side of generosity, and imagine that each member of that top 1% contributes an original piece of content on your site every week, whether that's a blog post, event listing, short update, photo or video. The 9% that are editors might each contribute a comment each week. To get 10 contributions a day, 5 days a week, you'd need a community of 50 creators and 450 editors...which implies that you'd have 4,500 lurkers for a total audience of 5,000 visitors per week. Once you consider that a lot of your lurkers will only visit once or twice a month (perhaps less), figure that you need to attract an audience of at least 10,000 regular visitors. So if you are doing a really great job of attracting interest from your potential audience -- and by really great job, I mean that 10% of the people who could conceivably find your site interesting do in fact visit your site on a regular basis -- then you need a potential audience of 100,000 to have even a remote shot at building a solid, user-driven online community.
But in a world in which 1% is considered a good click-through rate (i.e. if 1% of Googlers click on a link in your Google ad or search results, you're doing well) then it's probably more like 1,000,000. That's why Rob and I usually advise people to create an online community that's pitched at a niche interest (knitting for kids) or a geographical area (people in Vancouver) but not both (Vancouverites who are interested in knitting for kids).
This is the point where a lot of membership organizations will tell us why their members are different: they're more engaged, more passionate, more motivated, or more bloggy than the average. These members will bust the 90-9-1 rule, and contribute to the organization's site at unprecedented rates because this is the online community they've been waiting for, the network of colleagues they've been waiting to connect with. Back in the day (i.e., before Facebook and LinkedIn and Twitter) that might have been true (though we never saw it); these days, anyone who is really that eager to connect with their fellow members or professionals is already making those connections within established platforms like Twitter.
We're always open to the possibility of an exception. But here's the thing: if your organization is the exception, and you approach your social media efforts based on the 90-9-1 rule, your members will hop on board, your members will engage more and more actively, and your community will grow very quickly. On the other hand, if you bet on being that very rare exception to the 90-9-1 rule, and your membership turns out to be even close to typical in their lurking and participation rates, you'll have spent a lot of money to build something that can't really survive in the face of low participation.
So I recommend that membership-based organizations with a limited (under 100,000) membership take one of two strategies:
Develop a social media strategy for engaging your members that will be credible and useful even in the absence of member participation, but even better to the extent that members start contributing, or
Develop a social media strategy for engaging the general public that builds on your members' and organization's strengths, and serves your members by reaching out to a much (100k+) broader audience.