Even thoughwe haven't got a social networking site internally yet these are some things that have proven to work well for us or ones we're likely to impliment on our internal site.
Posts from senior management and heads of departments. This is a real biggie, get some of your execs to blog and people are really hungry for it. On an internal group authored blog site that serves as a news site for 1800 people the directors posts are consistantly the top viewed items on there.
Posts on different departments and what they do, people like this and we all wonder what other parts of the business do.
Wanted items and items for works well too, popular place our people look.
Blogs, group authored works well to start with.
We do activities related to work organised social events and also corporate social responsability events and blog posts on these in the group authered news site get a lot of attention.
Awards for staff and feedback from customers gets a good view.
Special interest group areas, for example we have a lot of people who like to cycle, other interests exist such as gaming and so on.
Hope this helps a bit.
Message was edited by: Iain Colledge Forgot to add, make it everyones homepage :)
We added a "Speak Out" community where employees can ask questions, make suggestions and comment on what is happening in the company. we have also actively found groups and executives who should reply to these. We have also added an area for social topics.
We already had a number of executive blogs, but I think the informal and more interactive discussions provides a better feeling of collaboration.