So I've been thinking about getting my hand on a starter set but before I invest money into this I was just wondering how popular is this game still? Is it dying out? Or is it still going strong?

Also when it comes to expansions. Are the cards inside for one faction only or are they for all the factions?

It's still going strong, but depending on where you live there may or may not be a local playgroup. The silence you typically see on these boards is the result of a couple of factors: most fans who post online about SWLCG do so on the fansite cardgamedb.com, and at the moment everyone is looking forward to World Championship Weekend and the upcoming deluxe expansion, Between the Shadows, which will expand upon the Jedi and Scum and Villainy affiliations.

Each deluxe set (including the Core Set) contains material for each affiliation; it's just a matter of quantity. 2x Core Set will get you a full playset of Rebel, Jedi, Imperial, and Sith sets to work with, while Edge of Darkness expands upon Smugglers and Spies (LS) Scum and Villainy (DS). Together, all six affiliations have a stable of nine useful objective sets. Keep in mind, though, that because these sets are built with the casual player in mind, to get a full playset of everything you will need two copies of each. Force Packs, on the other hand, contain 2x of everything that can be played in duplication, but do not all necessarily contain all six affiliations. The objective set system, while innovative, does lead to some asymmetry in this way, but I believe the game has about reached the point where it's only a minor issue among existing players.

Does all this make sense? Let me know if there is anything I can clarify.