Why will a member visit your community for a second time?
The first time was easy. Promise a big benefit, spike an interest and a member joins. It's the second visit which is usually overlooked.
You must persuade a member to do something in their first visit which will cause them to return for a second visit. e.g. Invite a member to give an opinion on an issue, the member returns to see what others thought of his opinion. Or invite members to send welcome messages to new members of the day. The member returns to receive the messages.
Ratings work well too. Encourage people to rate, critique or make suggestions on new profiles. HotOrNot thrived on this.
You can also take a mass-approach to this. If you host a newcomer of the week/month column/box, a new member is encouraged to visit again to see if they won.
These are a few ideas amongst many. It's not important that you use them, it's critical that you consider why a member will visit for a second time. Don't leave it to chance. Don't hope a member will suddenly think "hey, I haven't checked out that community I joined last week". Proactively plan the second visit to take place.