Maladaptive Daydreaming: where wild minds come to rest
It would be nice to get a precise handle on "significant".
Firstly, it seems reasonable to assume that there is no particular selection effect - that twins are not particularly more likely to join an internet forum than anyone else (unless the forum is actually twin-related).
A caveat to the above is that twin birth rates are different in different countries, and probably the membership of this forum is not evenly selected from the total world population. However, in US and UK, which account for most native English speakers, about 3.3% of people are twins. (Also, we could use the highest twin rate in any country as an upper bound for the probability of being twins, and a significance result based on that would still be valid.)
It then boils down to simple arithmetic, and a bit of statistics theory:
Going to the Members page, it tells me that N=4570. So the null hypothesis estimate for n is 150. In other words we have to find that there are significantly more than 150 twins on this forum before we can get excited about it (subject to the caveat that if we do a systematic survey, probably not all 4570 signed-up members will respond to it).