Basic GW

Im playing on factions, but theres nightfall and eye of the north, should I switch to a more recent one, like are they more populated than faction is ?? Ive read they have more quest, items, but more players or its the same ??

Overall, I'd say yes, Nightfall and EotN are more populated than Factions. They don't have greater number of quests and items, but EotN has some of the more desireable items and skills.

What do you mean by switch? You know that you can add all the campaigns to one account so that the same character can go to all areas right? It just sounds like you think that all the campaigns are separate games to me.
anyway, yes. I would add the other ones on to your account. More skills/items/quests/missions and so on to do.

By switch I assume he means take his character into the other campaigns to complete them before factions.

Personally I'd say it depends on your class. If you're a Ritualist or Assasin you should stick to factions, since you'll get the most drops appropriate for your class, as well as the most elite skills. Then I'd move to EotN or NF.

Any other class, I'd probably vote for NF, as it has inscribable weaponry, which is easier to work with, as well as other class appropriate drops. EotN I'd save for later, it's a pretty hard campaign if you're just starting out.