You're probably going to get lots of advice here. But, from what I've learned....you have to have LOTS of content and promote the site everywhere. You can have the prettiest and feature-filled site, but if you don't market it correctly, you won't get visitors.
I've used my RSS feeds and signed up at TwitterFeed. Then, that feed goes to my Twitter account and the Twitter feeds into Facebook.
Now, that also assumes that you are "working" your Twitter and Facebook accounts. Simply go through and find folks with similar likes (or dislikes depending on your forum topic) and 'friend' or 'follow' them. The more you have, the more others will 'share' or 'retweet' your topics and information.
Another big piece of getting a forum to be popular is to have exclusive content such as features, downloads or information available only to registered members. And, in order to help your community promote your site, run promotions such as "refer a member" for points. And, the highest point totals earns a prize of some sort.
This is the advice : Focus on your target keywords.
What I meant is don't try to own the web rather focus on let say 5 keywords to start, rank well on those then ad another and another ...etc ... Just make a plan and follow it.
Dedicate 30 minutes daily to facebook,
Forums must have good content AND good members Who are knowledgeable/good (sometimes sweet/good looking/hot models it really depends on your forum )
Not to forget Technorati, yahoo and your authority site (which depends on what kind of site you are developing)
... etc ...
xenforo will be a great source for forum administrators, so if your main target is us then you must advertise here. This is just an example ...
There is a lot to know about online marketing, in combination with running a forum. My experience being:
The minority of forums that begin will actually be around in 5 years time.
The minority of forums that are created will be successful within their niche.
The majority of all traffic to your site will come from Google.
A minority of traffic can be sourced via direct advertising upon other websites.
SEO techniques are not quintessential of running an online community. They play a small part only.
The majority of forums will only remain IF the initial first year, two or even three, the administrator puts in daily time to create conversation.
Content is king, but only when used specifically, not just to attract users, eg.
Placing RSS feeds in to create site content and garnish traffic is useless, because news stories rank in date priority normally, with the exception of a handful of highly valued established news sites, because they are literally replicated in their millions upon release. Absolutely useless.
News stories are worth if, and only if, you are completely writing the news story in your own words and only quoting specific aspects from a source. This makes new, original content vs. replicated news feed content done in the millions.
Create a small number of forums initially, extremely subject broad, and only create more forums when you have atleast enough specific interest in one topic to warrant such. Basically, too many empty forums and your site immediately loses impact to the new user who stumbles upon it.
Popular becomes only from a conversational forum that also answers a persons question. There is nothing worse than a new user who registers, you neglect their question or send them elsewhere for the answer... that doesn't retain users nor garnish popularity. Questions must be posted, they must be answered. When starting off, this means the forum creator.
Be unique... there is already plenty of people who are doing most topics... so you must be a unique voice with a unique opionion. A cult is often formed because the leader has a unique opinion. A forum is somewhat built under the same premise, being it must contain a unique opinion / angle that appeals. I a subject has a main forum that allows a, b & c... then yours must target d, e & f as a primary angle, as there will always be those who disagree with the current angle / site / site politics / rules / etc.
There is lots and lots more... most of which you only need to know once you have created the site and its functional.
My average expectations for a typical niche topic would be something like, 100 members in the first year, 1000 by end of second year, then exponential growth. It all depends on how broad or narrow the niche topic is. It could be 1000 by year one, 10000 by year two, if the topic / market was that popular. Those forums that see instant fame, they are the 0.0000000000000000000000000000000000000000000000000000000000001% of forums created. Sometimes the right idea just catches on and a forum gets that instant success... but its not something you can really look for. MySpace thought it was going to live forever... now its dead compared to FaceBook. AltaVista thought it had the search market sewn up... now most don't even know who AltaVista is as Google accomplished that space.
Popularity, being the word you used, is something you must measure based on realistic facts and the topic presented vs. existing competition. Even xenforo... if they didn't have those existing ties to VB, then that immediate market wouldn't even be here for an initial sale... they would just be another BB trying to make a name for themselves. If you have that name already... then certainly a forum based on that can create advantageous membership for an opening.
A global press release for a forums opening can do more good than thousands of wasted $$$ on other site advertising. One well written press release via someone like PRWeb, for less than $500 can create an instant buzz to any community and quickly find its way into being written about as a possible new buzz site for x topic.
I'll just tell how I did it. Since I seem to be doing pretty well it must be one way of doing it
1. post on other forums in the same niche for a while, build yourself a reputation on those (of someone friendly who helps anyone for free)
2. build up strong connections with members on those forums, if possible meet them IRL
3. when you have a reputation and connections, start to think about a domain name and how you want to set up things (technically & your principles...)
4. setup things the way you planned
5. let all your connections know about what you are doing, tell as much people as possible, if possible announce the news on forums and social networking sites
6. ask your connections/friends to start posting on your forums, tell them you have plans to grow it in a big community in the niche
7. post as much unique content you can. Let others reply to questions on your forums, if no answers, answer questions yourself!
8. try to minimize moderation, give people freedom to post whatever they want. But make sure you draw a line!
9. don't expect success in a few months.
10. expect possible failure or temporary slow growth, but never give up