However, asking members to opt-in just doesn't work. Success rates will be much higher using automatic enrollment with the option to opt-out, versus asking users to opt-in.
Granted, this evidence is from a Vanguard analysis of 401(k) plan enrollments, but I think it's a pretty good bet that similar behavior could be observed in members of an online community. The Vanguard analysis found that automatically signing up all new employees for the 401(k) plan resulted in a participation rate of 82 percent, versus a 57 percent participation rate in plans with voluntary enrollment. Those who were automatically enrolled had the ability to opt out, but the opt-out rate was low and stayed low over time.
The bottom line: people tend to go with the status quo and don't tend to opt out of something. And that was in a situation where money was involved - a more risky proposition than being included in a member directory. If you apply this same logic to a member directory, then it's reasonable to expect the opt out rates to be even lower.
I've only worked with a handful of orgs that have selected to use the opt-in approach, and they were mostly in Australia and Europe where privacy laws can be much stricter. Each org could not get more than about five percent of members to opt-in. When it was clear their strategy was failing, they took an alternative approach. They worked around it by including an opt-out as part of the member renewal process. Upon renewal, if the member did not select to opt-out, then they were, in fact, opted in. Coincidentally, only about five percent of members actually opted out as well.
Here's the thing: starting with a blank slate and depending on people to take action to populate a member directory, one person and action at a time, is a recipe for failure. Say you launch your community and send all members an email asking them to opt into the member directory. How many will actually open that email, then click the link, then take the time to opt-in and select privacy settings? Based on typical email open/click-through rates, not that many (unless you have an outrageous marketing and resources budget, and even then it's not a certainty).
Compare that to a scenario where an organization launches a community and opts all members into the directory, but includes the ability for members to opt-out with one easy click, or to adjust privacy settings to suit their preferences. Most people probably won't care about being included one way or the other, and those who do care will be able to opt-out or set privacy controls quickly and easily. Which member directory do you think will be more comprehensive, and therefore a better member resource?
Give your members the capability to control their own information quickly and easily, but don't depend on their taking actions to establish something as valuable as a member directory.