Re: Is this a problem?
As SteamID's follow a predictable pattern, and free accounts cost nothing but an email address to set up, and spamming a thousand users can be automated using Steam's WebAPI, yes... it's a problem. Maybe they haven't got to you but I've had a dozen or so and two just this week.
The hope is that you'll add them as a friend, then they send you an IM with a link. That link - if you're daft enough to click on it - goes to a credential-stealing page and - again, if you're daft enough - if you log into "Steam" on that website it gains access to your account. From there, it has another account that it can quickly (and probably automatically) spread from, can steal all your in-game items, stored inventory games, Steam Wallet cash, etc. and pass it off - again, automatically - to some central accounts that someone can flog all the gear from on the market and get payback instantly.
By the time Steam catch up, thousands of items are involved, there's thousands of transactions to revert, etc. and they may never know which ones were genuine and which were just people selling on that item they traded for. The big-name games have been stripped out of people's inventories and sold on (possibly even for real cash, Paypal, etc.), they have enough junk to craft expensive items via an automated API, and sell those one to unsuspecting users. You can even go to other markets (e.g. TF2WH, etc.) and trade things across different games etc. until you find something that people will pay cash for.
Given the setup, if you can eventually get even a couple of hundred quid, you've paid for all the infrastructure, steam accounts, programming (more likely script-kiddie downloads), etc. and if you're malicious maybe even cheated and got the accounts VAC-banned.
And all by just running a program that creates free accounts, automates some chats, trades and sales, and a weblink with some dodgy Javascript on it that plugs captured details back into the program.