It is right that not everybody is a member of LinkedIn. But the network is growing VERY fast. From 19 million to 32 million users worldwide in one year (2008) is a fast growing pace. So maybe the person you are looking for was not on LinkedIn yesterday, but signed up today.
Let’s also look at it from another perspective: a few years ago, when LinkedIn didn’t exist, it was almost impossible to find the paths between people. Or it cost lots of time. Now LinkedIn makes it a lot easier. And if you don’t find the person you are looking for, what keeps you from doing it “the old way”?
I repeat what I wrote before. Despite the fact that not every person is on LinkedIn, it is a website for business networking. What we see in practice is that the majority of organizations are represented on LinkedIn (In the USA, all of the Fortune 500 companies have an executive level presence). Maybe you won’t find the Marketing Manager of a company, but you might find the IT Manager. The Marketing Manager is only one step away from him. OK, it is some extra effort, but still lots easier than before LinkedIn existed.
To your success !