According to What use are Steam Trading Cards?, additional Steam levels grants extra friend slots. Given that I have as many Steam friends as there are fingers on my hand, I didn’t realize there was a limit in the first place, so in the interest of finding out how useful gaining a Steam level actually is, how many slots do each level grant you?
Answer
As per this Steam forums thread, and this Community discussion:
By default, you have a limit of 250 people for your friends list and every level adds another 5 slots – which means that you get 50 slots per milestone (10, 20, etc).
In addition to that, linking a Facebook account adds another 50 slots – to a base of 300.
Attribution
Source : Link , Question Author : Private Pansy , Answer Author : user98085