It is clearly their plan to do this, and in line with their stated intent to form the "identity layer" for the web. It's unlikely to work though, because the web has a great tendency to upset the plans of those who want to basically control the whole thing. It's sure that some kind of identity layer will come into use, but an open system (maybe a descendant of OpenID, maybe not) will always have the advantage of allowing myriad people and (importantly) companies to do different things with it. And that's the kind of evolutionary openness that saw off AOL in the 90's in favour of the web.

You know, another really obvious reason why it might not be a good idea:

Facebook ads are really cheap and have awful CTRs. Advertisers are used to paying very little for them.

Web ads though cost much more and are expected to perform a lot better.

What if FB launched an Adsense competitor, but no one cared because it performed so much worse?