I have gotten quite a few sign-ups from mailers with small numbers of members... I agree, I believe it's because they are new and activity is high at that time.
Funny, this was a popular subject of a post I did last week... Sparked alot of interest about where traffic exchanges have been and where they are going. Many different opinions, but a great discussion none-the-less.
Its here if interested in reading:
http://marcymcmanaway.com/old-time-traffic-exchange-marketing/