I also second this i would love to be able to tell when my friends are online so I can make trades with them instead of just DM them and waiting till they are on it would be amazing to tell when people are online or offline.
As long as there is also a toggle that lets you turn off/on the visibility of you account being offline/online (Kinda like Fr, which i hate to compare both sites; but i don't think any other pet-sites have this function), As i know some users probably wouldn't like the idea of a user constantly looking over their account or something! :0
With the ability to turn it on or off (the beloved toggle) I'm fine with this. I don't necessarily want it, but as long as you can opt out, I don't oppose it either.