Can anyone tell me what relation this fantasy grounds survey has to actual overall popularity of various games?
Well, not really, but their is at least some things that can be inferred. Generally if a system grows or declines on FG, then it is probably growing or declining in the larger market. BUT, maybe not... (see below)
I understand that. But what percentage of the total frpg market uses Fantasy Grounds?
Unknown. We don't even know what percentage of the VTT market FG has, and even less of an idea of much of the RPG market uses VTTs.
WotC posted a number of players sometime last year, but I can't remember the number so I won't guess. The only thing I fairly reliably remember from it was that females made up something like 40%.
That's reassuring. Thanks for that. So success on Fantasy Grounds is not necessary for a

frpg to be successful?
Absolutely.
There are many factors that influence the use of a specific system/ruleset on FG. One of those is how much content is available for that system withing FG, i.e. a dedicated ruleset and DLC. Another is like within any small community, a few people can make a "large" influence, by bringing in their friends and growing organically.
One system that benefits from both a strong ruleset/available DLC and an organic community is Savage Worlds. SW has an excellent community developed and maintained ruleset, and a bunch of licensed DLC that can be bought and used within FG. So, it's probably safe to say that Savage Worlds does not equal 5% of the general RPG market even though it does on FG.
There are other systems that have good community rulesets that are probably over represented (AD&D, DSA, CoC, C&C...) but other than knowing they have good rulesets and active users in FG, I don't have anything to compare to. On the flip side, WOIN has a ruleset on FG, but it's numbers don't show up because they are too small few. How that correlates to other WOIN sales/use would be up to ENWorld to elaborate on if appropriate.