Hi Bart, I am an old acquaintance from NUIM, just stumbled onto your site.

I am a twitter spammer, among other networks.
I thought that I would add some insight, perhaps explain some things.

"I laughed at the pathetic grammar, and at the concept that anyone was stupid enough to think they could make money like that."

I laughed when I read this, there is a good reason for this. The grammar is poor because the posts use synonym generating scripts to generate many different words to replace the original messages with. You can essentially send the same thing 100,000 times, without twitter noticing duplicates and the spambot going nuts. The price is poor grammar. In this industry, with money as the benchmark, scale trumps grammar everytime. This is one of our best tools against their anti-spam software.

Also, I make between 5k and 15k (EUR) per month on twitter alone. Put together affiliate network marketing accounts, many landing pages, and targeted spam (ex. your cat), and conversions from clicks to cash in my pocket is extraordinarily high. Yes, conversion rates are %0.1-0.5, but when you message 500k targeted people with a $10 kickback on all referred sales, its good money.
Takes a few hours of work, or less.
Then repeat it with automated tools many times per month. I put this detail in to illustrate my point, that people DO make good money with this. Don't assume they don't.

WRT: JC's points: Banning IP addresses is a non-runner. Of course spam is not sent from your own IP. huge private proxylists are generally used. Banning an IP will kill maybe 15 spam messages and 1 account. less, perhaps.
Checking keywords, I've explained that above.
CAPTCHAs: everything has been cracked already. Other software I use has cracked google and recaptcha, the two most difficult. These hamper beginners, yes, but not the top people who send the real volumes of spam.

RE: "because the only way to have spam-free email is never to give your address to people"
Just thought of two ways to counter this, I'm sure theres more, but just to be pedantic... :)
1) Name generators (doesn't really apply to you, as you do have a unique name, and presumably a non standard domain!).
2) Computers infected with malware, trojans or botnet clients: If somebody on your contacts list gets infected by zeus, for example, the trojan client software can be set to automatically pull every email address that it sees, and adds it to a database on the remote server. The infamous storm botnet is the best example of this.

RE: Fran (hi BTW I assume this is ringleadr)
Facebook has tons of spam. I am doing some work on facebook within the ToS, and I can see several variants of spamming. Zombie accounts, autocreated accounts with pretty girls profile photos which auto-add friends based on fake location/university/school, keylogged from trojan accounts (current estimates at almost 1 million facebook logins stored in the various botnets databases). There is commercially available software for spamming on facebook. The keylogged accounts are sold in batches of 1-10k logins. Those are the most valuable, because when you have an aged account with 100 friends, and many comments between you and friends, and interconnected friends, Facebook will not block the acct if your scripts add another 4900 friends, and keep updating your status message with links to your websites.

Generally, Bart does have the right idea. Do not realease your email to the public domain, and you're reasonably safe.