How Long Do You Wait Before Ditching a Dying Social Network?

We've all gotten ourselves so immersed in using an online service and social network, that we continued to use it even when we knew it was on the way out (stop pretending like you weren't checking your Myspace last year). Digg, post-acquisition, has now fallen into this group.

When you know a social network you use is likely on the way down, how long do you hold on to it before letting it go? A month? Six months? A year?