This is a common occurrence, actually. The issue is that Google AdSense requires a couple of things to work properly. First, you need to have your site up long enough for Google itself to know about it at all, obviously, and you need to have the individual page up long enough for the AdSense search system (which is different from Googlebot, the Google search engine bot) to see and analyze the page. Remember, AdSense is a dynamic contextual advertising system so it needs to actually read and analyze your page before it can properly target those ads.

The implication is that if your page hasn't yet been analyzed, the AdSense system is going to either show no adverts at all or show public service ads (PSAs, what you're talking about).

There are a number of other reasons why PSAs can show up, however.

The most common is simply that Google doesn't have enough adverts in the context network that match your page of content to fill up your ad block. If you're writing about, say, how to feed an aardvark, it's quite possible that there's not a single advertiser who has purchased an AdWords advertisement against that set of keywords. The result? PSAs.

You can also get PSAs if you have a keyword or topic that's on their restricted list: gambling, porn, or other topics. Google describe these topics as "could be construed as potentially negative, non-family safe or even offensive". Got that?

I've also seen where AdSense doesn't present PSAs, but presents generically targeted ads for a short period, for example "blogging" related adverts on a page about Google AdSense. After a while, the ads become more accurately targeted to that page of content, rather than the site's content or implied context. I think that's just a temporal artifact and I ignore it unless it seems to be a problem a day or two after the page goes live.

Another issue is that sometimes I've noticed that Google will decay to PSAs if you're requesting too many ads too fast. You can try to generate this: just keep reloading and reloading an AdSense-enabled page as fast as you can and see if you keep seeing legit and targeted ads or not. My theory is that this is probably some sort of click-fraud protection scheme on their part, but I don't really know for sure.

Finally, you might find that you're getting PSA ads simply because you don't have enough useful and meaningful content on your site, enough keywords or descriptive prose that the AdSense system can catch with its analysis hook. If you are posting YouTube videos, for example, with a sentence like "ROFLMAO!" then it sure seems to me that you're quite unlikely to have good adverts on that page because you simply don't have enough information for the engine to figure out what you're talking about.

The converse of this is if you have too many words -- a common problem on index or home pages -- then you can also befuddle the Google engine, because if you have dozens of topics, which is the best match for an advert? Worse, if you have very high keyword density (e.g. every 11 words you have the same key phrase) then your page might trigger a keyword spam situation and have ads blocked for that reason. I know, I know, you need to not have too many, but you need to not have too few too.

The solution? Model natural behavior. If you were to read your page out loud to an audience, would they have a clue what you're talking about (too few problem) or would they just keyword think that keyword you are just peppering keyword your speech with some keyword sort of keyword in a strange sort of way (too many problem)?

Anyway, hope that these ideas help you understand what's going on. I also recommend you check out AdSense's own page on the subject: Why am I getting PSAs?