How is the lower level population?
Pretty much non-existent, but I guess it can't be much better on the f2p server either, with the instant 60 thing it might even be worse there.
It's not a server-specific thing, the game just promotes rushing to max level ASAP. If you decide to play, I suggest you do it too.
Will it feel social or more like a single player game?
It can feel social if you join a guild, but social interaction will most likely be limited to talking in guild chat and occasionally a higher level player coming to help you with a quest that is designed for a group.