What type of advice? User submitted content and advice (http://www.reddit.com/r/relationships), or a site where you write articles about 'the top 10 secrets to dating every guy should know'?
It is a big niche, but it's also highly competitive, and you would be competing against people that are already well connected and established in the industry. I would say the chances of success are extremely slim.
Simple pickup are examples of guys who did it recently.
The darker, and more dollars, can be found at /r/seduction and darker still is /r/TheRedPill/
Every few years there seem to be a fad about these things, HN used to have these pick up artist posts pop up regularly until about 2 years ago. I got the impression that there was a large SV contingent involved in the pickup artist scene which eventually spawned the brogrammer movement, though no-one openly discusses it.
I know people who make $1 million+ yearly in affiliate commissions in this niche. There are many info products, membership clubs and networks with 7 figure revenue and few with 8 figure revenue.
As others have said, yes, it's big. But it's also crowded. And it's hard to be clearly better than the existing players. (Or at least, hard to be clearly better in the eyes of those looking for advice.)
But as with all existing players, there's always room for competition. Just look at Tinder disrupting the dating scene when there were already big names like POF, OkCupid, etc.
It is a big niche, but it's also highly competitive, and you would be competing against people that are already well connected and established in the industry. I would say the chances of success are extremely slim.