(I ran into myself on my second playthrough--thought that was nice, and I'd share it with you. ;] )
A online multiplayer game for a weekend game jam--that is superbly ambitious, and I must congratulate you for actually completing it! :) I also love the poem joke with the controls at the beginning--that's hilarious!
The gameplay is . . . ok. Moving on the ground is kind of fun. But everything I did outside of that felt . . . off. Double jumping feels stiff and awkward. And trying to land those jumps was not fun. I tried to reach the top of the screen, but I could never feel confident I was going to land on platforms that were only one square--and most of the platforms are that length. I fell a lot, and eventually gave up.
Here's the weird thing: during my first playthrough, I somehow missed the fact that you could double-jump, and single-jumped the whole time. One-jumps feel good to do, even if I didn't really get anywhere (most platforms were just out of my reach!).
Speaking of that, the controls and overall design of this game are both quite odd to me. I check out the xkcd comic games are based on before I play, and I liked the fact that the game that inspired Skateboard can be controlled with just one hand (arrow keys) and that the level is structured like a 3d skating arena (because it's for a hoverboard). The fact that you made it a skateboard is totally fine, I have no problem with that--but it's all designed like a platformer, with nothing to do with skating at all; take away the skateboard, and is the game really that different? And the controls are a bit obtuse, though the ideas behind them are great: the ability to put down your own checkpoints, and the goal of exploring the area to maximize your distances & put down geocaches are all great ideas with odd controls to go with them. Though I do have a question: what button do you have to press to return to your checkpoint flag?
Another thing I liked in the original xkcd game is that there's a goal: get the coins as fast as possible. As it stands right now, Skateboard doesn't really have a goal--yet. I can see in the description that if the game was fully finished, other players would be able to take your geocaches after you put them down. I really like this idea; it's a shame it wasn't implemented. Maybe after the jam is over?
I like the stats screen. Though typing in "NorbezJonesv3" (the third time I opened the game) made me put down a balloon when I pressed "3". Also please add the controls to either the stats screen or the description; they're easy to forget and strange enough that I really did forget them.
Overall, this is an ambitious project--I must restate how awesome it is that you made a online multiplayer game for a weekend game jam! I want to see the post-jam version of this, because it could be really fun. Nice job making this. :)