It creates a thread per member, our feeling is that it keeps it a bit personal.

Who is the thread starter for the birthday threads?

You nominate a userid in AdminCP settings. By default its 1 - admin.
We have it set to the site owner (Rick)

Kym

Tsk, Tsk, Kym. ;)

Is it possible to have this only post threads for members who have been active in the last x days? Because I can't imagine it working great with my 617000 members lol

It does that!

just set days at 21 or 30 and posts at 5, 10 or 50 (whatever) so only those with recent activity get a greeting

Thanks for this, going to give it a try.

Does the day period and post count work together or separate?

Say its set at 50 days and 5 posts, a members birthday is coming up and they have not been active since 2009 but has 20 posts. Does a thread get made because of the post amount or in this case does it not get made because of the set day time limit of 50 days?

The day period and post count do work together.

Ie. set 2 posts in the last 10 days.
This allows you to set a basic activity level.

It also only counts posts to forums that count towards 'post count' - if that makes sense. ;)

will this detect all user groups i have on my forum if i only exclude user groups 8

yes!