In the majority of cases its not a 'person' that spams forums in this manner - its a program that searches the internet for specific flavours of forum software and uses know vulnerabilities to post messages. One press of a button and the post is sent to hundreds of thousands of forums.
Its a numbers game - posting to 200,000 forums - some of them are going to stick.
Professional forum spamming normally looks a little different, posters will start with a few one word answers or some random questions - then add a forum signature or edit posts at a later date.
Forum software should always be kept up to date (although from experience this is harder than it sounds!)
As for someone spamming to your contacts list it could a number of things - perhaps sinister software on your computer thats harvested your contacts list or maybe you have accepted a rouge application request on facebook or something. There are some really bad facebook applications out there that are set only to obtain information from your facebook account and use it for their own needs. Dont forget if you done a 'friend-finder' on facebook - facebook knows the email address and name of everyone you have ever written too in the entire history of your email account - if your friends have public profile pages on msn, facebook or any other social network site extra information like telephone numbers, address, DOB's can also be easily married with email address.
As a rule of thumb - I never download any software thats not passed scrutiny from a well known company - I never let children or anyone else use my PC - I never accept any facebook application requests - I never use public computers to access passworded sites - I always scan my computer for viruses and trojans every night.
How to (try) stop it - I would install
http://www.microsoft.com/security_essentials/ run it now, and every night. Dont go to sites your unsure of - if you get any popups that give you a choice of yes and no - and you dont know what it is press neither, just the cross in the corner or just restart your computer/browser. Change all your passwords. Update all your software, ie windows and your browser, if you use msn chat update this too. Never use the same password for multiple sites.
I am ultra paranoid on my computer or any computer I use to access passworded accounts, there are so many ways thats its possible to hack someones account so so so many ways.
Ta
Baz