I'd advise putting your forum at example.com/forum, if the forum is related to the main site, and there will be interlinking (which there should be, if you want to benefit from the forum!).

Set a 301 from example.net to .com; do not use subdomains.

My logic is that for one topic, one site is your best bet in terms of all incoming links land at the same domain; all your marketing and promotion are from the same domain. Using two domains, or subdomains, simply dilutes your effort, and makes navigation more complicated.

I don't fully understand your concerns about advertising; it's your site, so long as you read the TOS carefully, I don't think you'll find restrictions too onerous.

Don't worry what 'very big' sites do; they don't (for example) need to worry about making example.com as large as possible - 'critical mass' matters, but not to the big boys, they've already got it! They need subdomains, extra domains, simply to manage the mass of content; smaller sites don't, and it weakens them.

Quadrille, Thanks for the comments.

As to the advertising, the reason I mentined is that some here said that some ad networks do not allow Forum sites to be a member, therefore if I have the Forum at example.com/forum, my site can be seen as a Forum site.

Is this true?

I haven't heard it, but I'm not an expert in the myriad advertising opprotunities out there!

Even if some weird company didn't want forum placements, few would extend that to a site that hosted the forum (barring sheer stupidity!).

But unless someone chips in with some knowledge, just be sure to read the small print - and work with a sensible advertiser ;)