The Leaderboard is present at the top-right side of the game. It shows the biggest players by decreasing order of their mass. After an update to Agar.io, the leaderboard became much smaller than it once was.
In the computer version, you see the top 10 cells and your own position, wether you are in the top or not. Your name will appear in pink-red. Players without names are designated as "An unnamed cell".
On mobile, you see the top 5, and what place you are in out of 100. Once you are in the top 5 on mobile, you can see who is in sixth place (#6). With commands and editing, you can set the leaderboard so that it can show countdowns or people who are winning.
If no people are on the server, the leaderboard will not show up. In the Battle Royale mode there is no leaderboard in general. If there are less than 10 people on the server, the number of people in the server will all be on the leaderboard (except if the mode is Battle Royale). This is what happens in Last Man Standing. If you win, you will be the only one on the leaderboard.
Often, people use troll names so others think that one place on leaderboard is missing. The name is, without the quotations: " "
Another troll name is "An unnamed cell". People seeing you on the leaderboard think you have no name, while you actually have one. It is also possible to use overlong names and names that have high characters that overlap other names in the leaderboard.
If you see a big cell not moving, check the leaderboard. Disconnected cells never show up on the leaderboard, so if this cell's name is still on the leaderboard then (s)he's just AFK, or pretending to be.