My friends never invite me anywhere. Help!!?

I have 3 close friends that are pretty much like my bestfriends. Everyday at school we always hang out and talk the whole day 24/7 and hang out on the weekends sometimes. But sometimes they never invited me. It sometimes felt like they were using me cause when we did hang out it was always at my house because i live on the lake and im a member at a private country club that has fun stuff to do. But none of them are. Now that school is over and its summer i havent got invited to do one thing with them. I can see like on twitter and facebook that they are always hanging out and they just went to the beach together and they didnt even invite me. They will text me sometimes and say "hey maddy! i miss you soo much! weve got to hang out soon" but then they never invite me anywhere. I invited them to the country club's pool and the lake to go tubing and boating and stuff but they always are like "let me ask my mom" and then they will say their grounded, but its obvious its not true. My mom is always telling me when they say something on twitter that they are together for me to say "thanks for inviting me" but i really dont have the guts and i dont want other people that follow me to think im a sore loser or something. What do i do? PLEASE HELP ME!!!