1. Why an I notified to a civilization's achievement when I havent met them yet?
You can assume that the UP is informing you of it, but this is particularly useful (and in fact borderline necessary) with Galactic Achievements as only one civilization can build them. As for the GAs that the races all tend to "spam" at the start of the TA game, each tech tree has unique starting GAs that only that tree can build which provide a marginal albeit not insignificant boost.
3. Why the heck is the modding community and forums so dead? With such an awesome game and the amazing customizing, i figured their would be more activity. Everyone moved on to Sins or something?
It's really not half as dead as you think it is. Check the Library link on the left for mods and check here for the most recent and active GC2 threads-although offtopic and some others are also accessible from the GC2 forums as well.
4. What is the future of this game? I take it no more expansions or big patches? Is Galciv 3 in the works? If so, is it going to be this year or like in 2012?
2.0 was supposed to be the final patch, but Brad has said at least twice or thrice since then that he can't stop making improvements to it, so we're relatively likely to see minor incremental changes (don't expect anything sweeping) in addition to bugfixes for anything gamebreaking that happens (there appears to be a diplomacy trade bug or two on the loose at the moment in addition to evil AIs frequently not expanding properly). But there is not intended to be for instance a 2.5, or perhaps even a 2.1.
GC3 doesn't have an official (or even unofficial) date as of yet, but 2012 seems to be the one that's kicked around the most, with some posts having said 2011. Others more aware of this kind of info will probably provide it. In any case GC3 is intended to be based off of Elemental's engine, which is in the works now. Impulse's site says it's due for release in February of 2010, so at a minimum don't expect even a GC3 beta before then.