Google will index your subdomains and count them in your site’s total number of pages, and large sites generally beat out small sites.
If you’re going to have a subdomain forum on your site and you’d also like your site to rank well in search, just by linking to that forum, Google will find it’s pages.
Things to consider:
- Make sure your robots.txt file is clean of content=”noindex, nofollow”
- Google does take its time
- Add a sitemap and refer to it in robots.txt. For example: “Sitemap:http://forum.example.com/sitemap.xml”
- Make sure the link to the forum doesn’t have a rel=”nofollow” on it.
- Check the site with xenu link checker
- If you have duplicate content on a subdomain, Google will avoid indexing it
Keep in mind that Google uses subdomains for its individual services such as news.google.com or maps.google.com. Notice that these are completely different in content and nature, and therefore perfect for specific subdomains.