Im looking for the best practice to let Google know about a vast number of dynamically created content. Let's say (I mean - dream) that I'm Facebook, and I want to let Google to index all the users' posts.

1. Sitemap.xml may be the answer for this but they are limited to 50,000 URLs in each site map. I know that I can create 500 sitemaps and create a sitemap for sitemaps, but they are also limited, 25,000,000 URLS sounds quite enough at the moment, but could cause problems in the future. I.E - stackoverflow already has 3 Million posts, probably sitemap is not the solution for them.

2. Creating a page with paging, and links to all the dynamic data. i guess this is what stackoverflow did by creating this page here: http://stackoverflow.com/questions

So I think that Option 2 is the answer, but it seems to me that sitemaps might have some added value. So what should i do?