Haven't seen you for weeks.

几周没见到你了。

Long time no see.

好久不见。

It's been months since we last met.

自从我们上次见面已经过去好几个月了。

Fancy meeting you here.

没想到在这遇见你。

I'm so happy to see you again.

又见到你我太高兴了。

You haven't been around much lately, have you?

你最近没怎么在这里露面，是不是？

How come I never see you?

我怎么一直都没见到你？

What have you been up to all these years?

这么些年来你都在忙什么呢？

Where have you been hiding?

你躲哪儿去了？

Hello stranger! Have you moved or something?

(好久不见)你好呀，你是搬走了还是怎么的？

注：hello stranger是对于认识却很久没见的人的幽默说法