Although, granted, given that Picard was more of a rake in his youth, it is a little unlikely that he was unaware of the world that was later established as the go-to pleasure planet of the quadrant even as far back as the 22nd century.
One would also think that such a planet would very actively advertise
itself, so Picard should probably be aware of it they way we today are aware of commercial products we don't necessarily even use...
Then again, the idea of "pleasure planets" was promoted in TOS already, and there might be plenty of competition in that respect. Perhaps Risa only becomes a favorite of our heroes (and thus a more common screen presence) after a few of them bring back good experiences, while a few others tell how prices at Wrigley's Planet and Disneyworld have gone up and quality has plummeted? Who knows which pleasure planet our DS9 heroes would choose to visit if Quark didn't receive a provision for pointing them in the direction of Risa?
It's hard to imagine most of the population striving for this and successfully keeping it a secret from outsiders.
Well, secrecy is a key part of the process even in the DS9 take on it. The people in charge of joining lie to everybody about compatibility issues; people who have already been joined may be in on the conspiracy (as our very own Dax was out cold and unable to comment in the episode where the conspiracy was revealed to the other heroes, "Equilibrium").
Just because being a Freemason or a Lion or an Alpha Phi Omegan or a Golden Pheasant or whatever is an advantageous social position doesn't mean that such membership would actually be publicly advertised, or that the society in question be marketed to the general population in any way.