Right now, steam user who refer 5/5 other steam users get a Steam Knight Mask.
So, people just invite those 5 people and do not bother inviting more people after that.
You realize that if you added more things than this, you'd get more players? Of course, not ALL of them would be active but neither are people who try out the game without being invited, they're not guranteed to stay for years to come.

I think we should have at 10 total invites, a Steam Knight costume armour, as well pumping out steam like the mask. I have no clue how it would look, I'm no designer.
And perhaps at 15 or 20 a Steam Aura?

Just like the mask, these items would be bound as soon as you pick them out of the mail so you can't sell them, so there's no profit to getting these items.

I know this suggestion doesn't mean much, there are more important things to do like the gun update and balancing monsters and weapons, new missions and all that, but give it a think?
You'd get more players, players from steam at that. Players who already regurarly spend money on games.