With over 4 million Mac X gamers playing on Steam, this change could affect many of those players. Valve said it made these changes in order to cut down on spamming and phishing schemes on Steam. It stated:

Malicious users often operate in the community on accounts which have not spent any money, reducing the individual risk of performing the actions they do. One of the best pieces of information we can compare between regular users and malicious users are their spending habits as typically the accounts being used have no investment in their longevity. Due to this being a common scenario we have decided to restrict certain community features until an account has met or exceeded $5.00 USD in Steam.

Among other things, people who have spent less than $5 on Steam can't send friend invites, use the web browser, access mobile chat or vote on Greenlight, Steam Reviews or Steam Workshop items.

Source: Steam