I've spoken with many web administrators over the years. All have mentioned the difficulty is not in driving traffic to their sites, but finding the "Stickiness Factor" - the concept of getting visitors to keep returning. This marketing term, coined in Malcolm Gladwell's book "The Tipping Point," suggests that the use of special techniques in packing and presenting material can render it memorable for the audience. To put it in layman's terms, it's the notion of trying to convert new customers into the "valuable" returning customers by providing them great products and services.

Note that it's quite difficult to pinpoint the exact stickiness of a message without research and experimentation, but I'll provide you with 5 general Stickiness factors relevant to the forum world...


Content
Content is the fuel that powers the engine. It is how individuals find, view, register, and most importantly engage in your community. Without it, there is little incentive for visitors to keep returning to your site.

To attract and convert users into returning visitors, produce threads/posts relevant to your targeted audience. Visitors to your forum are most likely there to seek for answers/advice, therefore become the "go to place" for information relevant to a specific niche. Try your best to answer any inquiries to the best of your ability.



Community
If content is the fuel that drives the engine, than the community must be the oil rig. This is the best advantage of forums - user generated content. The ability to create an atmosphere surrounding a particular niche and a place where individuals gather, interact, and discuss topics relating to similar interest is the basis of what a forum is all about.

Successful communities motivate online participation and there are numerous sociological factors as to why people participate, Wikipedia lists 3:
  • The Reciprocation Theory infers that a successful online community must provide its users with benefits that compensate for the costs of time, effort and materials members provide. People often join these communities expecting a sort of reward, whether it is physical or psychological.
  • The Consistency Theory says that once an individual makes a public commitment to a virtual society, they will often feel obligated to stay consistent with their commitment by continuing contributions.
  • The Social Validation Theory explains how people are more likely to join and participate in an online community if it is socially acceptable and popular.


Aesthetics
Design is important, but what is even more important is how you incorporate these designs around the main drawing points of forums, "content." Steve Jobs emphasizes the crucial role of perfecting the aesthetics of his product - why do you think Apple is now considered the most valuable tech company?


The use of proper fonts and colors will make your website much easier to read. According to w3schools.com, black text and blue links on white background is the best readable format.


To provide you a statistical conversion overview of the vBulletin's default style, the average time spent by visitors is 00.03:42 with an average pageview of 3.4 - very good.


Uniqueness
There are millions of websites out there - what makes yours different? In some regards, your forums functionality is just as important as the information it provides. Although I always advice vBulletin users about the "Less is More Philosophy," the vast amount of plugins available for download at vBulletin.org can definitely help provide the "sticking" point. Here are some that I've utilized to increase activity/time spent by users on my communities.
  • Post Thank You Hack -Light hearted peer to peer rewarding system.
  • ibProArcade - Arcade system for vBulletin and the most installed vBulletin plugin
  • vBulletin Mobile App - Alternative platform for connecting with your community on the go.
  • vBActivity & Rewards Pro - Incentive program in which users automatically receive awards based on numerous factors like posting, reputations received, join date, etc.
  • Envolve for vBulletin - An advanced Facebook-like chat enabling users the ability to instant message each other. I don't usually recommend instant chat plugins for forums, but this plugin contributes greatly to the average time spent by a user on your site.


Speed
The internet has unfortunately contributed to the exponential decrease in attention span for many individuals. That's why this decade is seeing a trend in emerging technologies emphasizing the importance of speed request. It's crucial for your content to be promptly distributed in a timely manner, else bounce rate will increase - making you lose returning visitors. It's also important to note that slow sites may be penalized by search engines.

With better browsers, more powerful processors, and improved technologies constantly releasing each year, it's becoming quite difficult to provide excuses for slow loading pages. Therefore, try optimizing your server for speed. Read up and learn optimization techniques at vBulletin.com's Server Configuration forum.

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In conclusion there are many more factors contributing to "stickiness." The ones I listed above are basically the general drawing points of forums. As I said before the best way to effectively pinpoint the perfect "Sticky" is by research/experiment. I can almost guarantee that your community will prosper under this proven theory.