How do they do it? Upvote memes for starters. If I like a meme, why not. It doesn't cost a penny. If I REALLY like it, I comment with a witty meme.
Look at the folks on the leaderboard. What do a lot of them do? They leave a comment as simple as LOL, a meme or GIF that says they upvoted the post, or other message. They have no problem paying the upvote tax. Why not? Because they know those comments pay back in points, but if a meme has a dozen commenters and no points, they would lose their credability.
But what do I know, I only got on the top 100 users less than two months ago, I only hit 1/2 million points this weekend, I've only had a few front page memes and GIFs. I only upvote 82 memes a day, comment 35 times a day, and meme comment 22 times a day. Oh, and post three memes a day. All since the middle of November. In less than a month and a half I hit 100K and had a front page meme.