In the mad scramble to collect new users, no matter how disinterested they are, current Facebook Platform developers sacrifice the most viral part of the distribution mechanism: making a highly-targeted invitation to a user who'll love the recommendation, and who will then spread the invitation to other highly-relevant users.
If Facebook Beacon takes the same shotgun approach to viralocity then I don't think it'll amount to much. If all the web service providers I use add the Beacon code to their templates and I spend the whole day hitting 'uncheck facebook friends' each time I perform an action on another website, I'll delete my Facebook profile quicker than you can say, "f***book".
If, on the other hand, Beacon handles the opting-in and opting-out in a smarter way (by, frinstance, letting me create short lists of friends with similar interests so I can manage opt-in/outs conveniently) then I think it could be huge.
Let's wait and see...