Feasability: The use of nicknames can only mandated if it is just and reasonable, so even the law here is not absolute. No one would require the use of pseudonyms from ebay or amazon.
Strangely enough, I have been using pseudonyms on eBay for like, ever.
Also, it is not about a requirement of Facebook users using pseudonyls, but about allowing the use of pseudonyms.
And it would be no problem for FB to allow that and support that technology-wise. So feasability is absolutely NOT a problem.
They basically only need to find a single feature that wouldn't work with pseudonyms and anything but real names isn't justified anymore.
Or they just block this one feature for people using pseudonyms.
Plus, since FB can hardly make sure that a name that I enter when creating an account is my real name, they can not really claim that their service wouldn't work with pseudonyms. I'm pretty sure that 10+% of the current accounts don't comply with the real name rule.
So this isn't about technical issues, this is only about Facebook changing their TOC (for German users, at least).