Networking is not about direct transactions, it's about getting to know people. Nevertheless it might be a good idea to network with people who have access to your target market. This could be the key factor in deciding which platforms to keep on using, after trying out a bunch of them.
On Ecademy you will find lots of small business owners, while Xing seems to have more production companies on board. The corporates are present on LinkedIn while you'll easily find the younger generation on Netlog. On Facebook you'll discover a nice sample of the total community, perfect for launching new products or ideas.
If your professional activities are aimed at a small and very specific piece of the market (e.g. lawyers), you might want to be part of a niche network for that target audience too.
Of course each network also has a strong memberbase from it's own origin. Ecademy was founded in the UK and still has a large percentage of UK members. Xing was launched in Germany and still has a huge amount of German members. This also gives you the opportunity to have a quick and easy access to the market in that country.
Some time ago I was helping one of my contacts to find a high-tech company in the neighbourhood of Edinburgh (Scotland), ready to accept an internship from a highschool in Belgium.
Because I knew Ecademy was probably the network with the strongest UK memberbase, I posted a cry for help on their blog and two days later the problem was solved.
If you want to do international business with a certain country, it's always a good idea to locate the top networkers in that country and meet up with them. They probably have all the contacts you need to get started.
Of course the best way to use online networking is to make the perfect combination of multiple networks, covering all your needs. More about this coming soon !