LinkedIn has said today it has now crossed the 100 million member mark – a nice milestone, and a good headline for a company that is probably going to be publically listed this year.

LinkedIn has released a whole load of statistics about its membership, showing that nearly 18 million people have troubled themselves to join some kind of professional group on LinkedIn and that 1.2 million posts are made by members each week. Quite impressive (especially if you’re under the impression that no one uses it and it’s all about Facebook and Twitter these days).

How has LinkedIn, which went live in 2003, avoided going the way of MySpace and Friends Reunited and other once-popular social networking sites?

LinkedIn has made a rather lovely infographic giving its side of the story. But it was hard not to notice that I suddenly got a LOT of requests to, well, link up with people, just as the economy was going belly up and workers were facing up to the reality of large-scale redundancies. Anecdotal evidence, of course, but LinkedIn’s figures back this up: about half of its 100 million members have joined in the last two or so years.

But before all of that, it was still pretty popular and, reportedly, profitable (in spite of its ads performing poorly). And apparently useful for things other than finding work, such as the “accidental entrepreneur” Henk van Ess.

LinkedIn is vowing to keep on growing until it reaches its aim of “connect[ing] all of the world’s professionals to make them more productive and successful”.

If the global employment situation continues the way it has been for the last couple of years, as well as what seems like a longer-term trend for there to be fewer people in jobs for life, it might reach this goal quite quickly.