Most dating sites rely on advertising and premium memberships and live on the margin between advertising costs and revenue from membership fees. This is a hard business - not many reach the status where they make more then 1 Euro on every Euro they spend on advertising. Also, it is hard to get members to sign-up on a dating site and not every "expensive" visitor through a sponsored link (e.g. Adwords, Facebook) will result in an active member and not every active member will end-up paying something.
Now that the financial crises reaches many countries, also people are less willing to spend money on online dating. This might kill many dating sites that already had a hard-time trying to make more money on membership fees then spending on advertising, customer support and everything related to running and maintaining the website.
Another danger for many dating sites is Facebook: in real-life many people meet their partner through friends. Now with Facebook, it becomes even easier meeting friends of friends and Facebook also helps singles to meet their partner directly or indirectly. People also have a limited time to spend online every day - the more time they spend on Facebook - the less time they will spend on other websites such as dating sites.
Facebook, however, might become victim of its ipo in the near future - the stock market can be a friend of a business or an enemy.