I use Yahoo, Outlook, Gmail, and LinkedIn. I have 2000 Outlook contacts, and 300 LinkedIn contacts. I went through the import process, and of my ~2500 contacts, none were Oovoo members. There is clearly no critical mass yet. To build a critical mass, you need to open everything up.
The first, second, and third rule of building software for use by communities is "Don't tell the community what it wants." As a person who has been involved in both failed and sucessful software development projects, I can assure you that you will gain much and lose nothing from defaulting a new user to invisible to a search, but allowing them to change this and making those who have opted in visible to each other. I cannot fathom why you would project your personal preferences on a large group of people with an opinion different from yours - particularly when it would benefit your company not to do so.
The community wants a directory so bad that your most prolific poster here (TheBoss) is desperately trying to build one himself. This is typical of what happens when developers ignore the needs of their users. It is still early - don't make this mistake.
Adam