They are doing ok AFAIK, but are almost two orders of magnitude smaller than FB, user wise. I don‘t think Xing is even doing that well, they have started very aggressive monetization tactics recently, like forcing people to purchase a pro account in order to view incoming friend requests.
I don't know about _strong_ network effects, but part of the appeal of Facebook (for me) is that it's universal - visitors to the country (e.g. exchange students) and people I've met overseas (holidays) are all in one place. Further, it doesn't result in you being introduced to (possibly better) Facebook alternatives in those situations. It may not a strong effect, but there is definitely small cross-country effects IMO.
