[logfromblammo]

That is literally the first time I have ever seen the tilde used in that way. I have seen it used to indicate sarcasm, satire, or snark on at least four separate web sites. In any case, we already have a mark for surprise or excitement: the exclamation point ('!'). No doubling up when we still don't have a generally accepted mark for insincerity.

[cthalupa]

I imagine it's a bit of a cultural thing, so it'll depend on the sites you visit.

I've seen it uses regularly as an indicator of positive sentiment by Japanese netizens, and people imitating them.

[thaumasiotes]

How did you get "surprise or excitement" from "general positive sentiment"?

[logfromblammo]

It goes the other way around. The exclamation mark is supposed to denote surprise or excitement. General positive sentiment is represented more closely by '!' than sarcasm or satire is represented by anything.

There are only a few symbols on the default US keyboard that are potentially eligible for promotion to punctuation. If you want "general positive sentiment" to have its own mark, why not use '+'? Plus for positivity. Tilde for not exactly this, which is close to its use in mathematics.