One reason the smartphones like the iPhone are gaining ground on purpose-built cameras is their instant connectivity: Take a photo and you can immediately share it on Twitter, Flickr, Facebook, or other popular sites. Wouldn’t it be great if your SLR or digital camera could do the same? This is the promise of the Eye-Fi card: It adds Wi-Fi connectivity to most popular cameras, enabling you to transfer photos directly to your laptop or the Internet. If only it worked.