You should never let the words “I told you so” come out of your mouth.

Why?

• It does not build up or lead to anything. In most cases, these words follow after someone tells you how something didn’t go as planned. You saying ITYS will not change the situation, nor does it help the other person do better next time (they already learned that by failing) and it won’t improve your relationship either.
• It is not pro-active. If you’re living in a western country (you probably are) your style of communication will mostly be “transmitter oriented”. This basically means that it is the responsibility of the speaker to communicate ideas clearly and effectively. If you want to blurt out “ITYS”, you are probably referring to an earlier conversation you had about the same topic. Did you do everything you could to help the other get started back then? Did they know the context? Did you check in along the way?
• At Facebook they have a powerful saying that “something is never someone else’s problem”. While this might not be applicable to this particular situation, if you’re so invested that you’re commenting like this, why didn’t you fix it yourself?

Start replacing “I told you so” with more conscious, productive words and questions to help another along.