。
Making friends is easy, but keeping friendships is hard.
Literal
Friend [object-を] make [nominalizer-の] [topic-は] easy [but-が] friendship [object-を] maintain [nominalizer-の] [topic-は] difficult.
A parallel structure: two nominalized clauses (Xのは...Yのは...) set up a contrast via が. たやすい (容易い) is a slightly literary synonym of 簡単 for 'easy,' giving the proverb a polished feel. 保つ (たもつ, 'to maintain, preserve') is more formal than 続ける.