Don’t you want your charitable efforts to do the utmost good they can in the world?

I suspect this is going to put people on the back foot. (as in - a defensive position. Making it hard to take in the rest of the article) Especially as its a rhetorical question coming from the negative.
Try:
"So you want your charitable efforts to do the most good they can in the world?"

imagine

I know plenty of people for which this word is a turn-off if not used carefully. knowing as well that some people don't imagine like other people means you need to be quite careful with the placement of the word imagine.

my personal perspective

sounds like it's purposely weakening the quality of the source. "it's just me saying this"
remove "personal"

really passionate

remove "really"

I have taken the... pledges

there's an implicit boast in this statement; and an accusation towards the reader as to why they have not done it yet. That's not a good attitude to be spreading, and probably won't motivate people.

identify as...

keep your identity small ... identifying is something we shouldn't recommend.

four of GiveWell’s top choice charities for 2015.

Top for what? include the relevant information. Without the information; it's more fluff than value sharing.

A widely-used general estimate is that for every 1 person who shares an article, 100 people read an article thoroughly, while many more skim it.

source?

This article could do with a number of improvements...