It is an interesting niche. I can see the site is totally blank from user content at the moment, as even the top 5 hotels listed on the home page have 0 user reviews.
One thing I struggled with is actually how you write a review.
The instructions tell me to click on the 'Write a review' button and then search by either hotel name or city before selecting the best match.
So I started typing the city, 'London' getting hotels for London, Ontario and London, England. However, the results only gave me 6 results in London, England and the hotel I was looking for wasn't on the list (there are obviously a lot of hotels in London).
At this point I didn't realise that you could type in the hotel AND the city and that it didn't matter which way around. It was only through trial and error that I found this out.
The instructions on the home page say Hotel Name OR City when actually it is better to search using Hotel Name AND City.
The write a review page does actually say search by Hotel Name and City but by that point I had in my head that it was Hotel Name or City.
Back onto User Reviews - you really need to get the ball rolling with at least some reviews on the site so you can show users hotels that do have reviews. Having a totally blank site is obviously not a good thing.
I would start asking everybody you know for a review of a hotel gym to get some online. Get friends and family involved, a lot of them must have been to a hotel that had a gym even if they didn't specifically try it out. Try to get 20 reviews, then 50 reviews so that when you begin using other methods (such as asking people on forums, twitter, etc. to submit reviews) they are not faced with a totally blank site. I wouldn't want to be the first person to submit a review on a site.
Even if people don't have the time to give you reviews, discuss the review with them, make notes and then submit it to the site yourself to build up the numbers initially.
Once you have 50 or 100 reviews on the site then start your other methods of getting the site known. Like Mike says, find the forums and sites where your target market hang out. Get your social networking moving as you are doing. Partner with existing review sites like Tomas mentions.
But the main point for me is to make the site appear a lot more active that it currently is to start the ball rolling.
Hope that helps.
p.s. on a side note, how the hotels ranked when there are no user reviews. Surely they would be ranked by a score based on the number of positive/negative reviews. But when there are no user reviews then the fact that 10 Manchester Street Hotel London comes out as the number 1 hotel in London, what is this based on?