I'm working on a dying dwarven civ that had a small pop but no sites. I built the population up with a few fort embarks, then did some adventuring to a) clear their ruins of inhabitants and b) cause their conquered sites to become ruins by killing/recruiting the occupying population. Now the plan is to get a king and general in a fort located within the civ's territory. Once that's done I hope to see the civ reclaiming and/or founding new sites while I pass time in adventure mode and other embarks. I'll be surprised if it works.
Observations while all this has gone on (25 years in-game):
* a human civ lost a few sites and ~20% of its population. Its law-giver and general were stuck in a perpetual state of leading attacks on different sites. The stasis may be broken though; checking legends after 5 years in my current fort, the general successfully captured the forest retreat after lo these many years; the law-giver died and was replaced, then replaced again, the latest one is currently showing up at my fort as a diplomat.
* a tiny human civ launched a few founding expeditions (at least one of them happened while playing adv mode, though it's likely to have been during the 2-week time advance)
* goblin civs (two in this world) had big population increases, expanded like crazy, conquered some sites from the first human civ and elves, did that goblin thing with the trenches and towers in those sites (again while I was in fort mode)
* elven civ's activity parallels the first human civ, no pop gain or loss. They're at war with everyone and constantly launching attacks with a few successful site captures. No founding expeditions, one reclamation attempt. (Fort mode.)
All that said, I don't see evidence for much if any world activity during adv mode. Considering the slower scale and time consuming nature, probably I simply haven't spent enough time in adv mode to see anything happen,