The thing with plenty of fish is that it takes NO investment whatsoever since it's free. So a lot of girls throw their profiles up there and just do it for attention. Even my own sister put a profile up once and said she had no intention of dating any of these guys, she just wanted to "see what kind of guy I could get." So she was doing nothing but trying to inflate her ego and feel validation and nothing more.
If you are going to go the online route and take it serious, probably best to look into a SERIOUS site where people aren't allowed to put up free profiles. That way, you KNOW any woman that's on there is serious since she's invested her own money. Having a pay system screens out a lot of the flakes like my sister just looking for validation.
As for the race thing, Ok Cupid did a very interesting study on this issue:
http://blog.okcupid.com/index.php/2009/10/05/your-race-affects-whether-people-write-you-back/
Black people get pretty low response rates, but the thing is, men get low response rates across the board. Even white guys only get a response of 29%, compared with just under 22% of black men. The average reply rate of women to ALL men is only just over 27%, whereas 42% of men respond to women's contacts. This is of course just one site, I suspect on some other sites like POF which are filled with attention wh0res, female response rates are even lower.
Finding girls online is cool, I know guys that have found some really awesome girls online, and one of my best friends met his wife on match.com and they now have a house, a kid and another on the way. So if it works for you, it works for you. If you aren't getting good results, try changing the profile pic, the writing, show it some people to get feedback. If it still doesn't work, then forget it. Move on and learn face-to-face game. Online dating just isn't everyone's niche.