It started by bolding my own milestones, then Scott's and other moderators if I recognized their screen names. Then for awhile, I bolded milestones by The Exceptions members from the folding race.
Afterwards, I'd sometimes bold a milestone by a long-time team member that I recognized. A sort of extra callout for sticking around.
A few months ago, I decided it wasn't fair to new members, of which we have had so many. I don't recognize half the names in our member lists today, but their contributions to the projects are just as valid.
I'll probably go back to bolding my own milestones and maybe other mods. Or maybe those of people who contribute news to the team site.