To answer this question, I’d first ask you what your goals are with online dating services and how savvy with computers and online dating you are. Those who are looking for superficial flings obviously have different options than people looking for long-term commitment. Also, people who are very comfortable with online etiquette and the culture are different than someone who’s new to computers or the dating game.
Market research I’ve seen indicates that most people, given a choice, will say they’re open to both casual dating and long-term relationships. In other words, people sure seem keen on keeping their options and opportunities open. Assuming this, my general response would be not to recommend an individual online dating service. Rather, I urge daters to join several online dating services. You see, relationship science tells us that most people make rapid judgments about the "suitability" of romantic prospects using just two criteria – how attractive a person is and how accessible they are. Let’s call this the "attractiveness and proximity" principle.
Therefore, one great way to increase your chances of finding a large pool of quality prospects is to broaden your exposure to attractive people who live reasonably close. Shopping at only one grocery store really limits your opportunities of meeting people. Likewise, using only one online dating site limits your opportunities for finding people who you’ll find attractive and accessible.
Today, the choices in online dating services are immense. Daters can use this to their advantage. Don’t be fooled into thinking that bigger dating sites are always better than smaller sites, or that paid membership sites must be better quality than "free" sites. I’m not saying a person should join every site there is, but having profiles on three to four different types of sites can be a smart and successful approach. I’d recommend trying one or two larger sites, and also posting profiles on one or two niche sites. Niche sites tend to be smaller sites but that cater to specific interests – like "Republican dating" or daters who share a "love of dogs or cats." Niche sites can also focus on a particular religious orientation, which is a very important "deal maker or breaker" to some people.
Niche sites can be wonderful for those new to the online dating scene, because they offer an instant connection or natural topic of conversation among people. It really helps take the pressure off. On the other hand, large sites can also be wonderful for newbies by providing the opportunity to learn how to use some of the tools of online dating that smaller niche or free sites may not offer, such as video chatting or advanced blocking features to help filter romantic prospects.