From Diablo Wiki
This sort of feature will be (Blizzard assures us) optional; players do not have to link up their Battle.net accounts to their Facebook account, or connect their real name/identity to their Blizzard game play. This was the bone of contention during the Real ID debacle, when fans were outraged at Blizzard's plans to require players to use their real names to post on most of the Battle.net forums, including the Tech Support forum.
Battle.net and Facebook Integration
"We're pleased to be working with Facebook to integrate their platform with Battle.net to enhance the social-entertainment experience for our players," said Paul Sams, chief operating officer of Blizzard Entertainment. "This new functionality will make it easier than ever to connect with friends on Battle.net and play StarCraft II and future Blizzard Entertainment games together."The new Facebook functionality in Battle.net will be tested in the near future via the ongoing StarCraft II: Wings of Liberty beta test and will be available to all StarCraft II players when the game ships later this year. Information about other Facebook-related features on Battle.net will be announced at a later date.
This announcement was greeted with more suspicion and worry than excitement and anticipation, but the attachment of publicly-viewable real names to Battle.net accounts didn't become a big issue until later, when mandatory plans for Real ID in the B.net forums were announced. (Then scrapped, after massive fan opposition.)
Facebook Integration Quotes
Jay Wilson answered a question about Diablo III's planned social media intergration during an interview from Gamescom in August 2010. 
- Gamona.de: What Battle.net features will we see for Diablo 3??
- Jay Wilson: We’ve just started working on Diablo-specific feature set for B.net. No strict determinations yet. Anything you see in SC2 or WoW we’ll use it if it translates into D3.
- Gamona.de: Including the social integration and Facebook options?
- Jay Wilson: Yes, it’ll all be in the Diablo 3 Battle.net. If you’ve got a friends list set up in Starcraft or World of Warcraft, it’ll be waiting for you when you start up Diablo 3.
Anyway. I still have yet to hear any logical reasons for why providing people the option is a bad thing.
A fan asked for more details about how much information would be provided, and whether participants could toggle on/off the automated updates.
That’s the extent of our Facebook integration at this time.
There is a feature on the World of Warcraft armory website where achievements can be shared on your Facebook page, and this has been and always will be optional for people should they want to use it.We can’t put anything on your Facebook profile without your specific consent by signing in to an integration feature with your Facebook login credentials. So, unless you do that, there’s no way for us to automatically or without your knowledge start sending things to your profile.
There seems to be something of a generational, or perhaps an occupational gap between Bashiok and the others at Blizzard, and many of the fans of their games. As Bashiok points out, he and almost all of his friends, including Blizzard employees, all use Facebook. They've grown up gaming online and using social media, and they don't think anything about keeping their real life and their gaming pursuits separate. Also, the fact that they get paid to make games features in there, since it's their job.
Many of the fans of Blizzard games though, even those who use Facebook and other such programs, don't want everyone with access to their Facebook/real name/real life to know what they're doing online. Especially not how many hours they're playing video games. Playing games isn't a problem for Blizzard employees; it's their job. But for a lot of other people, it's not something they need everyone who can view their Facebook page to know about. People might be playing online from work and don't want their boss to know, or playing more than they want their family (spouse or parents) to know, etc.
Players who do not want Facebook intergration, or do not want it turned on for them, offer various reasons.
The most common is from people who do not use Facebook, or wish they didn't, and just dislike the existence of the program.
Others who do use it are concerned about having their real names attached to their online gaming activities. This was the main thrust behind the opposition to Blizzard's planned Real ID requirement for forum posting. It's especially an issue for female gamers who want to avoid online harassment and stalking, or just don't want to be treated differently from other players, as they inevitably are if their gender becomes known.
Other players don't necessarily mind people knowing they're playing, but they do not want to send or receive automatic updates on it via Facebook. As most users of Facebook have witnessed, it's nice to hear what your friends have to say, but app spam is maddening, since it's inevitably generated the most by the people you are least interesting in.
As of now, with Blizzard promising that Facebook features will remain optional, most fans are accepting of the feature, and some are quite enthusiastic about it.