I don't find judicial activism when SCOTUS takes a precedent setting case to its logical conclusion. There's no question that CU opened the door to more money in elections, but that decision in and of itself doesn't constitute activism. The activism was in the original precedent (Buckley) to the extent that it equated promoting a particular candidate for election with promoting a political idea.
I'm not someone that likes to throw around the "judicial activism" label, as I think both sides can claim that in many cases depending on their particular perspective. I think what makes Citizens United particularly notably is that SCOTUS decided on its own to move the case from a simple statutory issue to a constitutional one that has had profound ramifications.
The Buckley ruling on self-expenditures by wealthy individuals was certainly a constitutional and not a statutory ruling. Since CU flowed from Buckley, how does one interpret it as a statutory to constitutional shift?