One way is to pay users to generate content.
Back in 1999/2000, I was CTO of a startup building a website aimed at independent travellers (which we defined as anyone who wasn't on a package holiday - i.e. people who were booking their own flights and hotels, and picking their own activities). Our plan was to become something like what Wikitravel is today - a kind of crowdsourced travel guide, with recommendations from other travellers for places to eat/drink/stay, things to do, sights to see, etc.
The idea was that a traveller would come to our site, search for, say, places to stay in Dublin, and the list they would get back would consist of recommendations/reviews submitted by other travellers.
We faced the exact same chicken-and-egg problem the OP describes - we had a website based on user-generated content, with no content.
People who were travelling/backpacking around the world during their gap year were a clear part of our target market and, at the time, there was no mobile internet, so gap year travellers would use internet cafés to access their email. The biggest and most popular internet cafés in London were run by easyEverything, where, on any given day, you could find dozens of travellers and tourists.
So, we did a deal with easyEverything to buy terminal usage in bulk, and we then gave away £1 worth* of usage to anyone who wanted it, in return for them writing a piece of content for the website.
We literally had people, wearing branded t-shirts, at these cafés, grabbing people as they came in and asking them if they wanted some free Internet time. From our perspective, it was a very successful approach - we were getting UGC for less than £1 per piece of content and, at the same time, publicising our site and getting a bit of brand awareness.
Unfortunately, the company didn't survive the Dot-com Crash but that's another story...
* I say "£1 worth" because easyEverything used demand pricing - if the café was empty, that £1 might buy you 90 minutes, if it was busy, it might only buy you 20 minutes.