+1 to @sillysaurusx's comment. I'd like to add on a recommendation to read The Cold Start Problem by Andrew Chen. It goes into the trials and tribulations of creating a product dependent on network effects.
More than just a problem of no longer having an "artificial" influx of users via HN, you will need to figure out how to build what Chen calls an "Atomic Network" of users who are self-sustaining on the platform. The first step to that end will be figuring out how to attract the "hard side" of the network (i.e. people who find good papers and post them). What keeps them coming back?
I'd highly recommend considering what the smallest network of users would be that could sustain something like Paperlist and what features they might want.
Some questions that might be helpful to get started:
* What incentive does a given user have to return to the site?
* How would a user be drawn back to the site, e.g. RSS, notifications, etc.?
* What incentive does a user have to contribute to Paperlist over submitting to HN, reddit, etc.?
* How do you attract a new user the to site? Put another way: how do you increase the "virality" of Paperlist? E.g. why would someone link to Paperlist today instead of Arxiv?
I hope this is helpful.