[AlyssaZara]

I would like to start a niche forum for example, dogs, cats, dairy industry, etc, but my question is, will it be profitable and make me earn big money from adsense or how many members do you need in your forum to earn atleast 500 dollars every month.

[MSL]

  Well, it's hard to say, because it depends...
1. If there is a lot of competition in the SERP of Google, Yahoo, Baidu, Yandex, etc, your forum will be almost invisible for the people who search this niche (for example most of the people will see page 1 and page 2... but who will see page 12 or 23 if your forum is listed there?)
2. Yes, there are other sources for visitors/members like Facebook, Twitter, etc., but there also depends of what is the situation with the competition, how skilful you are in SEO/SEM, how interesting and unique is your content and so on.
3. 500 dollars every months... well, it depends of how many visitors do you have, what kind of topics/threads you have, how long time the visitors/members spend in your forum, how often they watch the ads, where are the visitors/members from (which country)... So, it's not a simple formula. In other words -- some forums (and other websites) may earn USD 500 per month with 500 members, but other even with 3000 members can't earn USD 5 per month.
 I hope I was clear. And I'll be glad to read some other opinions.

[英语课]

I would like to start a niche forum for example, dogs, cats, dairy industry, etc, but my question is, will it be profitable and make me earn big money from adsense or how many members do you need in your forum to earn atleast 500 dollars every month.

A lot.

[SEO]

 Did you start your niche forum?

[jaelajens]

The question is very tricky, earning money from adsense is not that easy especially from forums since you need to moderate them now and then, I suggest to just create niche blogs instead.

[MSL]

  Oh, yes! I prefer niche blogs, too.