Steam is limiting the Community-functionality of users that haven’t spent at least 5 dollars on the service. With this measure Valve hopes to stop abuse such as phishing and spamming.
This limitation includes sending invitations to friends, opening group chat sessions and vote on Greenlight, Reviews and Steam Workshop parts. Valve announced this on its site. In addition, users with a limited account can no longer participate in the Steam Market, publishing reports on Steam Discussions is limited, they have no access to the web-api and can no longer use browser and mobile chats, until they spent the minimal amount.
The measure is intended to protect users against spam and phishing.
Malicious users often use accounts that has not spent any money. We can compare the spending pattern to distinguish the mainstream of malicious users.