我看Twitter最早有140个字的限制，后来国内做微博也纷纷效仿，这有什么典故吗？为什么是140个字呢？
Twitter 野史 记载着下面这段话：
And since the worldwide standard length of SMS (or text messages on phones) is 160-characters, the founders of Twitter thought it wise to stay within that bounds so as not to inundate people’s phones with 3 or 4 staggered, delayed, or even partially missing 4-part messages.
140-characters was chosen as a good length, leaving 20 characters for the username of the sender. This way, anyone receiving a tweet via SMS would get the whole tweet in a single text message, with nothing spilling over into a second or third message that pops up minutes later.
也就是说，Twitter 早期是为 Web 和手机间的信息通讯而设计的。过长的短信会被运营商分割，因网络原因导致收件先后错乱也时有发生，一条短信就能说明白多好啊。一条手机短信的长度支持 160 个字符，创始人觉得保留 20 个字符给发件人署名比较合适，于是就剩下了 140 个。
刚好两条短信的字数。这样就可以通过短信发推了。