Sadly from trying to use them, I'd say most of the 'swinging' sites like most of these dating and sex ones are absolute rubbish now.
Unless you really want to feed the ego of vile overweight pigs or skanky looking meth heads, steer clear and save your time. 1000's of men for a few women who are in reality ugly as sin, or fake profiles made by the site to keep traffic up. in 2015 your chances as a single man for an actual reply, much less a shag are faint to non existent now.
Saying that I do indulge regularly but not in the main swinging 'scene' or on those sites. The way I started doing this was because me and my wife had 'the conversation'. The promise was that if I could find a suitable female, that she also fancied she'd have a threesome for my birthday. That was all the encouragement I needed. Found our first playmate late last year. Chatted online, then we all met for a drink and a meal, week later she came to our place for what turned into an amazing night. We've been doing it ever since on the regular with a selection of great women we first met online.
My best sites for success are Craigslist (though it can be very annoying at first) and OK Cupid which I've gotten the most success from. There are a ton of very attractive polyamorous minded women on there, who are very open to engaging in threesomes, with the right couples.
Win/win for me.