Even I, a prime dinosaur, have been infected; last weekend I was at an engineers' reunion and wanted to post a "look what I'm doing" note to the engineering group. I only had my iPhone; with FB it's a couple of taps - I wouldn't even countenance attempting to post a photo to the Yahoo group.

In my view a group must have a mobile version or it will die. More and more people possess only mobile phones or tablets. IMHO you will have to cater for them with an iOS and Android app.