I have been playing with quite a few social network sites in the past (Xing, wkw, LinkedIn, flickr, last.fm). An issue I have with XING in particular is that with a free account, XING does not allow me to export my connections (at least I did not find a way to do that).
After Sebastian convinced me (and, apparently, a few others too) to create a profile on Facebook, I was faced (no pun intended) with the same problem again: How do I synchronize all my contacts between the various sites? After all, most of my professional contacts are on XING. But how can I invite them to Facebook without manually entering all names and addresses again?
This time I decided to solve this issue. I created a Greasemonkey script that steps through all pages of my XING contacts and puts a list of email addresses onto the last page. This allows me to copy and paste them into e.g. Facebook.
Here is the script: xing_export.user.js
Disclaimer: I only tested it on my personal profile, and only with the account language set to German or English. If you give it a try, please let me know how it worked out.