Sharding isn't why you aren't seeing so few people, think about it a bit:
If your world has split into 2 instances and you're only seeing 2 people as you play in it then:
1) If the instances have the same number of people in them, you'd only have seen 4 people assuming the same distribution, if the two instancs were joined into one. Hardly an increase to "massive".
2) If your instance is lighter than the other, say only 10 in your instance and 100 in the other then why aren't you switching instance so you can see 20 instead ( 10 x 2 )?
The issue isn't the shards, its the server population being spread over a large number of difference spaces. The areas are massive and there are a lot of areas. You don't see people because they are spread out.
Even inspite of there being not a lot of others visible on my screen, it is still an MMO in my view because:
1) There always seems to be someone close enough to click the clickie that spawns once every couple of minutes while I'm fighting the mob that guards it, forcing me to kick my heels for several minutes waiting for the clickie to respawn.
2) Some bonus mission bosses "kill leader of whatever" always seem to have a queue of people waiting for the next 5 minute spawn. Some are happy to join up to share the next kill, some are not.
3) I can still form groups for group content if I want to. Last time was yesterday at 4 am when there were only 6 of us on Balmorra.