Here is an incredibly easy way to increase your mafia size and is not by any means an exploit.
In some of the "add me" threads on the Mafia Wars fan page, sometimes people will post e-mail lists of people who are looking for more Mafia Wars members. With this list:
1. Click on the "Invite Friends" sub-tab under the "Friends" tab at the bar found at the top of Facebook.
2. On the "Invite Friends" page, simply copy and paste e-mails off of the e-mail list into the first box and type "mafia wars" into the second box. As mentioned below, I would only recommend pasting about 40 e-mail addresses a day (will result in about 20 friend invites).
3. Since these e-mail addresses already exist in the system, Facebook will automatically send them a friend request for you (you can easily send 20+ friend requests with the click of a button this way).
4. Someone posted a list of a few hundred people in this add-me thread, but there are tons of other e-mail lists floating around out there in various Facebook groups.
It is as simple as that. Set your Facebook status and that is all there is to it. I would not recommend adding more than 20-30 people a day with this method - any more than that and you could be flagged for spamming.