I kid, of course. While the ACM is a popular punching bag in hallway conversations, they do help the CS community in many ways, not the least of which is the digital library, the conference management services, the lobbying efforts in Congress (where I think though that the CRA has them beat), and so on.
But there's this constant running joke that the ACM is always trying to drag us kicking and screaming into the 1970s :). More importantly, while the DL has been spiffed up with comment feeds, all kinds of social sharing and what not (but no RSS for comments - COME ON !!), I think that the ACM could use its considerable weight to really help the research community. In both of the ideas I'm listing, the ACM has particular advantages as an organization with brand recognition, and an imprimatur of authority. They also have resources far beyond what other groups might be able to do.
A Pubmed for CS
More than six years ago, I was complaining about the lack of a Pubmed-style single location to dump all CS papers (titles, abstracts and keywords, with links back to original source). While the arxiv is becoming a better and better place for people to post preprints, what we really need is a single point to view and browse published papers, across journals and conferences.
The DL already maintains paper listings across a variety of publications, and many CS conferences are run by the ACM, so it shouldn't be that hard to do. The DL itself isn't quite there yet, mainly because of the lousy search features that it has, and because it's not quite complete.
I don't know if Dan Wallach's proposal for a central hub for papers is going anywhere, but that's another model that the ACM could help make a reality.
A Mathematical Reviews clone
This one is possibly more localized to theoryCS, and requires more resources. But it's getting ever more important. It's really hard to keep track of the flood of papers showing up in conferences, journals and on the arxiv, and a service that generated short reviews of papers would be great. Plus, as theoryCS gets more fractured and more diverse, this kind of thing becomes ever more important.
It seems like the ACM is getting the 'social bug' if the DL redesign is any indication. I'd argue that these two items are probably the best kind of 'social web' that the ACM can contribute to.