First of all, let me say that this not an official headline or press release from the Church. That says a lot right up front.
Also, these links are to either personal blogs or rumor sites (I can't really tell which). There is no official news organization reporting on this rumor.
I feel I also need to point out that Elder Ballard's talk does not specifically state to join Facebook. He mentions the name Facebook early in the speech as an example of what is going on today in the modern web. Then later in his talk he makes this charge to the graduating students of BYU-Hawaii a year and a half ago:
Now, to you who are graduating today and all other faithful members of the Church, as you graduate from this wonderful university, may I ask that you join the conversation by participating on the Internet, particularly the New Media, to share the gospel and to explain in simple, clear terms the message of the Restoration.
So with that in mind, I don't see Elder Ballard leaking some info that the Church is interested in Facebook. He was speaking about the "New Media" very generically. This is actually the definition of "New Media" according to Elder Ballard's speech:
Today we have a modern equivalent of the printing press in the Internet and all that it means. The Internet allows everyone to be a publisher, to have their voice heard, and it is revolutionizing society. Before the Internet, there were great barriers to printing. It took money, power, or influence and a great amount of time to publish. But today, because of the emergence of what some call New Media, made possible by the Internet, many of those barriers have been removed. New Media consists of tools on the Internet that make it possible for nearly anyone to publish or broadcast to either a large or a niche audience. I have mentioned some of these tools already, and I know you are familiar with them...
One of the tools he named specifically was Facebook, but I don't think this is an endorsement of one tool over another. This is the point I'm trying to make.
I personally have no clue why the Church would ever be interested in acquiring a social networking site like Facebook (especially one of the largest, if not
the largest). You can participate in Facebook as a member of the Church with no restrictions from either party. What benefit would the Church gain from acquiring Facebook? The Church certainly isn't interested in any financial gains from such an acquisition.
Also, I don't understand why Facebook would sell to the Church, just to let the Church narrow the scope of the application. I don't see why the Church would sponsor non-Church related conversations and networking. The Church would want to narrow the scope of the site to just members of the Church and/or Missionary Work as you suggest. But I could not see the Facebook developers feeling alright about seeing their product become more restrictive and go a completely different direction than they have already taken it. The only thing I could
possibly even imagine in my wildest brainstorming would be that maybe the Church would be interested in hiring the developers of Facebook to build a whole new site just for the Church and it's membership....but that's quite a stretch of the imagination.
I see no reason to take this rumor as fact at this point; there is hardly any evidence that I could count as credible. It is what is, just a rumor for now.