Valve has enacted a new policy on Steam "as a means of protecting our customers from those who abuse Steam for purposes such as spamming and phishing" -- accounts that have spent less than $5 USD on the Steam store (or the equivalent in foreign currency) are restricted from communicating with other players.
Until and unless $5 is added to the account (it doesn't have to be spent -- just reside in the account) these features are blocked:
If the balance dips below $5 -- meaning if it's withdrawn, or charges are reversed -- the account will go back into restricted mode. If the money is spent, it's still credited to the account. Activating Steam codes or retail games doesn't count, by the way.
The policy should go some ways to combating spam on the platform. You can find out more at the Steam support site.