Because you didn't have enough fans/followers on your Facebook and Twitter account. In my personal opinion, you need to have at least 100k fans on your FB page to have good reaches and visits to your sites when you post links on FB page and at least 10K followers to have good impressions for each tweet on you post on Twitter.
Exactly.
Go with Adwords, at least you need to have a good budget and spend $1000/month to have minimum sales for your sites.
Well, our budget was not $1000/month but even the low one we had it was just not worth it, and we were told that if we do better by referrals not to waste money on AdWords. Especially when competitors will happily clock up clicks and Adwords system is not detecting that, which isn't easy due to proxies and VPNs, etc. It can't be trusted 100%. For that reason, I don't think Adwords is that good for small to medium hosting businesses, IMO.
Of course forums are easy to drive traffic and promote your brand name for free if compared to others.
Well, I would not say just for free. I've spent a good amount on advertising at many different forums and find it more effective than Google Adwords. This is most likely a hosting industry thing.