I've been in the same place! First I spent way too much time going to every individual page and thanking every person. Seriously, hours. Thankfully, other friends have been far more clever than I have.
The one status update note is nice, but people may or may not see it. I've had friends do both of the following that have worked for me -
- gone to every birthday post on their wall and "liked" it. That way I knew they saw it, and they responded casually. Quite fine for the casual-ness of FB.
- gone to every birthday post on their wall and commented with a "thanks" or response to the specifics of the note.
I've also seen combos of those. I think the "like" works well for the people that just wrote "Happy Birthday", and sometimes longer notes are better served with a response in the comment line.