A support post on SteamPowered.com shows that Valve are planning to restrict Steam account functionality of anyone who's spent less than $5 directly through Steam:
We've chosen to limit access to these features as a means of protecting our customers from those who abuse Steam for purposes such as spamming and phishing.
Limited users are prevented from accessing several features on Steam, including but not limited to: Sending friend invites; Opening group chat; Voting on Greenlight, Steam Reviews and Workshop items; Participating in the Steam Market; Posting frequently in the Steam Discussions; Gaining Steam Profile Levels (Locked to level 0) and Trading Cards; Submitting content on the Steam Workshop; Posting in an item's Steam Workshop Discussions; Accessing the Steam Web API; Using browser and mobile chat
You will need to spend at least $5.00 USD within the Steam store. Some examples of purchases that grant access to these features are: Adding the equivalent of $5 USD or more to your Steam Wallet; Purchasing game(s) that are equal to $5 USD or more from the Steam store; Adding a Steam Wallet card to your Steam account; Purchasing a Steam gift that is equal to $5 USD or more from the Steam store (Receiving a Steam gift from a friend doesn't count)
Using serial keys to activate retail purchases on Steam will not count towards your account value, neither will activating Steam keys obtained from 3rd party websites (e.g. Humble Bundle) or games gifted to you by other account holders.
(Score: 0) by Anonymous Coward on Wednesday April 22 2015, @10:24AM
But I guess PayPal has your real name, right? So if you were doing bad activity on Steam, there would be a way to find out your identity by getting the information from PayPal. It certainly is possible if the behaviour is bad enough to involve law enforcement, but possibly it works already on a lower level through contractual agreement between PayPal and Valve.
And I'm sure that a prepaid card is also associated with a specific person.