As you guessed it, it would likely be hit and miss since you can’t control the content that users post. If you have activity on your front page I’d recommend not including more than 5 entries and having very short excerpts. From an SEO standpoint, you should have your headings set as your major keywords and include relevant content under your headings.
You want to choose about 10 keywords for each page and be sure to try to include them in your meta description which should be about 150 characters.
The internal links from your activity won’t do much for you, but they are important as long as the search engines can determine your sites structure. Try getting more external links to your posts, groups and forums. That will help you more than anything.