Thanks for comments AAA M8. I have tried to answer most of your questions here.

Question: "How many of those "48 mm users across the globe" do you imagine Microsoft ALREADY has access to (US excluded)?"
Answer: I don’t have access to exact numbers but, around 30-40% of traffic on Facebook comes from international markets. Here is why I think they needed access to those users.

Social Networks are different than portals and search engines:

Having access to users on portal or search page is different than social networks that thrive on User Generated Contents (UGC). Marketers need to measure the ‘user behavior’ of ‘target audience’ to be more effective. For example when a user searches for something the engine knows the user intent. Also, when a user goes to a news page the portal knows what he/she is reading. But, on social network it is difficult to target users with ads that is more relevant to the content he/she is about to generate. So, it is not just about having access to them but also about having access to them on the platform they are more engaged with and making ads more relevant.

UGC (User Generated Contents) is the future:

The current trend shows that user visits and engagement on portal sites are down then a year before. But overall, people spend more time on the internet. Both of these are contrasting numbers. But, if you connect the dots you'll find that Facebook and other social networks are growing much faster and are more engaging than portals. Most of the people who use social networks are of age group 18-35. But, Microsoft doesn’t have a social network.

Question: You seem to perceive that MS needs to gain "much needed momentum in online ad space"..Why?
Answer: Because MSN unit is making losses. Online advertising is the way to make money.

Question: Why would MS need facebook to help them fine tune their newly designed tools." ..?
Answer: Simple answer is – data is everything and since they don’t have any social network with such a large user base they desperately needed it. It is a virtuous cycle. More data translates into better algorithm and better algorithm means better results and better results mean satisfied users who in turn will generate more data. But, here is a much more complex answer.

As I mentioned earlier, the portal strategy worked well in the past but, it is gradually being overshadowed by User Generated Content (UGC) on sites like Myspace, Facebook, Orkut, Friendster, Hi5 etc. These are the things that traditional banner and contextual ads has miserably failed to monetize. It is a very big challenge for all advertisers to be able to efficiently monetize UGC. With all respect to Microsoft; they are NOT among the best advertisers when it comes to ad monetization. (And, even Steve Ballmer admitted that MS is a 4 year kid competing against seasoned pros that are out there for 10-12 years). These challenges are similar to what all major search engines faced in late 90s. Overture figured it out how to monetize search; but, it was Google who had access to 60% of the searches on the internet at the time (because of its sweetheart deal with Yahoo from 2001-2004). By the time Yahoo acquired Overture and started monetizing on its own; Google had made its natural and sponsored search tools far more superior than anyone on the block. This was largely possible because of the data it had access to while it was tuning its site. Even today, it makes more money per search than anyone. This has given them unprecedented advantage over rivals such as yahoo, live, AOL and Ask (both AOL and Ask use Google to monetize their search).

In the nutshell, social networks are going to be hot properties in coming years just the way portals were in late 90s. Monetization of a social network is very different and difficult than portals and search engines. User behavior in social networks is essentially a social behavior which is indirectly influenced by number and type of connections. It is much more complex to measure and analyze. Microsoft needed access to this data on user behavior. Microsoft just bought target audience by investing into Facebook. It still needs to figure out how to measure ‘user intent’ in complex social networks.

Question: AND finally, does the merit of this investment really depend on how well facebook is able to monetize itself?
Answer: I still think valuation of $15 billion for Facebook is too much. But, Microsoft isn’t paying $15 billion. By investing $240 million, it is just testing waters. If they are unable to monetize well than the valuation will come down.