That just takes one software developer to create the right application. I'm surprised it hasn't happened yet.
I think we might be past the timeframe for "surprise" and into the timeframe for "bemused resignation." The interwebz have had ample time to fill this gap and they have either chosen to not do so or found that it is impossible. I tend to think it is the latter. No matter how intuitive and accessible the app, the posts will still be made to the topic, and not to you.
Fundamentally, a community on Facebook or G+
is a discussion forum. It has a subject, members post topics related to that subject, and other members reply to that topic. They are inarguably less robust, but that's because no one is a member of Facebook or G+ for the communities. They're on these services for the convenience that is their "wall."
All of their posts go up on their wall. All of their friends' posts go up on their wall. The posts from any communities they are a member of are posted on their wall. Can you imagine what a member's wall would look like if ENWorld were a Facebook community? I can, because I'm already a member of enough G+ roleplaying groups that my wall is essentially useless. It populates so quickly that I'd have to watch it constantly to catch everything. It's not that I'm not interested in the content; it's that it's not organized in a useful fashion.
And it's that organization that slows forums down. Makes them more challenging to interface with. As [MENTION=697]mearls[/MENTION] said earlier, Facebook and Twitter and G+ aren't about in-depth back and forth. That loss of focus on trackable discussion is like a warp drive.
I think [MENTION=177]Umbran[/MENTION]'s observation is apt -- social media creates the illusion of community, but fundamentally you are friends with whom you are friends with and interested in the things you are interested in. Social media does not easily initiate change in those categories. You might send the second-chair violinist of your high school orchestra a note congratulating her on her newborn, but you're not going to her baby shower. She's a number in your Following/Followers readout.
By the same token, you are a member of a social-media community because the overarching subject interests you. Other members will occasionally post things of value to you, but you aren't there for other members' comments or Likes on those items, you're there for the item itself. Membership is fundamentally just a way to personalize your profile and collect recipes for rugelach.
I'm not saying discussion forums are the better form of social expression; I think the ship of public opinion has sailed on that one. I'm just saying they're different, and that I'm not sure the app you describe is a possibility.