I've never, ever seen anyone using the
@ sign with that intention. A lengthy search on the most popular social networks also did not yield any results (in fact, using
@ on them has proved itself quite problematic).
The only two reasonably common colloquial usages of this sign are:
For mentions to another individual, as it is usual in English-speaking countries (@Tyler Durden);
For gender-neutral nouns and adjectives, since most of them in Portuguese are gendered. This is a rather recent issue raised by modern feminists, for political reasons. Even amongst them, this habit is not unanimous, and many of them oppose it. From the Manual para o Uso Não Sexista da Linguagem:
Enquanto a linguagem continuar carregada de estereótipos, não convém
dissimular a visibilidade das mulheres. Por isso é importante evitar
as barras diagonais: ”oferece-se trabalho a costureira/o”. Não se
devem usar parênteses “buscamos um (a) advogado (a)”. Nesse mesmo
sentido é preciso eliminar os símbolos que não são legíveis ou que não
é verdadeiramente representação do feminino: querid@s amig@s ou todxs
A rough translation would be:
While the language remains loaded with stereotypes, it is not
appropriate to conceal women visibility. Therefore, it is important to
avoid slashes: "waiter/ress needed". On the same page, it is also
essential to remove signs that are either unreadable, or not a true
representation of women: "@he" or "xhe".
Keep in mind this book is not condoned by most linguists and authorities in Brazil, and it is merely meant as a guide for activists and the general population.
Whenever Brazilians feel the need to emphasize something in written colloquial language (assuming bold, italic, colored, and larger font sizes are unavailable), they'll either
CAPITALIZE IT or
!!! spam exclamation marks !!!. Using the @ sign with this goal would not be understood by the majority of the population, and would most likely go unnoticed.