Nick O'Neill at All Facebook reports that Facebook has confirmed that "all Open Graph-enabled web pages will show up in search when a user likes them." He also calls this Facebook's "war on Google."
While utilizing likes and the open graph as a ranking factor in search should help Facebook improve its internal search, it doesn't represent much of a threat to Google search. Google indexes the web. Facebook indexes activity from Facebook users. There's a pretty big difference, regardless of how big Facebook is.
There is certainly something to be said for Facebook search, however. There's no question that a lot lof people are using Facebook and spending a lot of time there, so having some kind of search strategy for Facebook is not a bad idea. Naturally, the Open Graph will play a huge role in this, and that means taking advantage of Facebook's social plugins. As I've written about before, Facebook likes (as well as Twitter retweets) are like the new links in some ways.
Facebook is definitely making a lot of moves to keep users getting the info they want from within Facebook. Fan pages essentially turn Facebook into a news reader. They're working on a Q&A product. They're launching content destinations themselves (like this politics page). However, no matter how much information Facebook is able to give users, that amount will always be limited, and will not be able to deliver the web in the way Google can. Of course, that's why they have Bing results for web search.
As far as search market share, it is probably Bing that stands to gain the most out of improved Facebook search. I don't know how often people are going to go to Facebook for web searches, but the more people do search on Facebook, the more they are going to see those web results from Bing, when the actual (limited) Facebook results don't deliver what they want. If Bing can deliver what they want in the top three results (the amount that is commonly displayed in Facebook search results), Bing only stands to gain.
Optimizing for Bing is very connected to optimizing for Facebook and soon optimizing for Yahoo.