NEW YORK ( TheStreet) -- Amid the BlackBerry's shrinking market share in the U.S., it appears that one admittedly small constituency is holding firm, and it's not the big banks, the CIA or some other arm of the federal government.

It's the beautiful, famous people -- Hollywood's A-listers.

Granted, BlackBerry's market share with those people isn't going up. Having been close to 100%, how could it? But the decline appears to be less than in the U.S. population at large.

This may seem counter-intuitive, but there is actually a very specific explanation for it. It's all about privacy and security.

Two weeks ago, Apple ( AAPL) announced iMessage, which appears to be positioned to compete against BlackBerry's Messenger product. The main purpose here is to wean people off the notoriously expensive SMS ("text message") plans, in which users pay an exorbitant amount per bit sent and received, compared to email and Web surfing.

There is no doubt that Apple's iMessage product will eat into BlackBerry's market share, because incrementally it narrows the gap in terms of their respective messaging capabilities. However, Apple's iMessage product is unlikely -- by itself -- to cause the A-listers to switch.

Apple's iMessage product is based on email and phone numbers. Basically, if you have someone's email or phone number, you can now cut cost and enhance the messaging features.

Here's the problem: A-listers don't want to give out their email or phone numbers. That's why they use BlackBerry Messenger (BBM), because BBM can be set up using only the BlackBerry-specific PIN number. Anyone who wants to enter your BBM system must first be approved, and you can kick out anyone you want, as well as block multiple requests. At no point do you need to let anyone know your phone number or email address.

In today's social networking-centric world, one significant source of disturbance comes from various classes of stalkers, paparazzi and clingers-on. A-listers need every weapon they can find to fend off the undesirable. One significant shield in terms of protecting against these kinds of threatening leeches is to avoid having to give out phone numbers and email addresses. As such, Research In Motion's ( RIMM) BlackBerry is a rare refuge of privacy for the A-listers.

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