First, thanks to TW of Why Don't You Blog? for this post telling readers about Planet Atheism. With more posts like this, the PA community can't be stopped. :)
And now, some considerations about the growth of PA and its relation to search engines such as Google. Besides being an atheist / humanist, I'm also a Linux sysadmin, among other things, and I've long been interested in search engine optimization (SEO), or how to make a site perform as well as possible in search engines. In fact (shameless plug), I'm the author of this series, for instance... and one day I may even finish this one. :)
How does this relate to PA? Well, remember that I said, in the previous post, that PA is getting 200 readers a day. Sounds good, right? Well, it isn't. After 4 months, the number should be much greater.
Here's why: PA gets almost no hits from search engines. Since every post includes a link to the original version (on the member's blog), search engines recognize (correctly) that those are the original content, not the version on PA. Therefore, if someone searches for something you've blogged about, your post will always appear in search results before PA... if PA appears at all, since its content changes daily, and isn't archived.
PA does get hits from people searching for "planet atheism" (duh!), "atheism blogs", and similar stuff, but these are much rarer: anyone who knows about "planet atheism" probably doesn't need to search for it, and nobody searches for "xxx blogs", they search for actual subjects.
In conclusion, PA gets very little help from search engines (much less than a "normal" site, or than any blog). This is why it's so important that member blogs (and possibly even non-members) help tell people about PA, since that's its only real source of new visitors. Remember, by helping Planet Atheism, you're also helping your own blog (if you're a member), helping PA get even more interesting as something to read yourself, and helping the "cause" of atheism / humanism.