Business social network Xing's membership reached 4m at the beginning of September, helped by its acquisition earlier this year of two Spanish networks.
Xing has posted some pretty impressive user growth figures recently, announcing in March that its membership had grown by 149% to 2m at the end of 2006.
With the acquisitions of Spanish social networks eConozco and Neurona, Xing said it had managed to further boost its membership. Both networks have continued to add users, with eConozco doubling its membership since the acquisition.
The company said the membership growth could also be attributed to an increase in the number of email invitations sent out by existing Xing users.
According to a recent survey (PDF) by the social network, 87% of respondents have recommended the network to friends and colleagues.
Rival LinkedIn, arguably the biggest 'professional network', surpassed the 10m mark earlier this year. New members are joining LinkedIn at a rate of 130,000 per week.
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