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I am planning to do something about the grow of members of my board. We still get new members, nearly 2-3 per week, but it should be more. So ideas, shared experiences, anything about "What can I do to get more (active) members?" are appreciated.

I thought about facebook advertising for example. Any experiences with it? Good or bad?

Nope, no experience...
My gut feeling is that facebook may bring members, but not many active members.

No real experience with the Facebook route, I'd guess that more promotion or posts on FB that lead you to click onto the forum will help with joins, but that does not necessary = active members.  But certainly fresh content on FB with more available on your site should entice one to check it out.

To me its:  content (the king),  easy to use (elkarte ), community (friendly, helpful, informative). 

I know on my one forum I get a lot of traffic through twitter, just have to use the right tags.

Content is king. If you have compelling content, people will find you.

On facebook you can create a page for your forum or a group. A group it does not have much sense as it is more similar to a forum, I choose a FB page for my community. It does not bring me users, but it is useful to comunicate with my users. It is fast, easy and free. It took me some time to get users to my page, but worth the effort.

On my forum I have a topic with a poll "How did you meet us" with these results:
- 54% searching for a dedicated forum for this car brand
- 35% searching info, opinions, response to a problem for a car model
- 2% a friend told me about this site
- 8% other

@Jorin I think your forums has lot of ptential, the hibrid /electric car is a market that will grow much in the future. Also in Italy there is a forum for that and has more than 3000 users and more than 300.000 posts. Keep continue posting good content and you'lll see the results.

p.s. when I started my forum skoda I had a few uniques article that I write and until a few months ago were on my top10 most visited topics. Only a few comments on those topics, but lots of visitors

Oh, we have facebook pages already. 

May be there is  no need for visitors to register. Offer some goodies that are available only to registered users.

For example? At my board only registered members can see some boards (marketplace, meetings and chitchat). So only them can join meetings and benefit from contests with prizes. I made a news message explaining this in the news section, so it is shown for guests.

We have such a poll @radu81 mentioned too. It says:

64% search engine
27% link on other site
9% personal recommendation
0% facebook and twitter

So I deleted our facebook and twitter sites. Have no time to post there on a regular basis. 

Social sites does not bring users on my forum, but is the easiest and fastest way to comunicate with members of my page which usually are also registered on my forum. It's a good alternative to newsletters and "less invasive"