I would suggest these three options:
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(1) Bist du Mitglied (a) in (b) vielen sozialen Netzwerken?
(a) "Mitglied" means "member" <> "Anteil" means "part, share" (noun) but you can't use it with regard to humans.
(b) "in" is needed here
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(2) "Bist du Mitglied vieler sozialer Netzwerke? - This is the genitive form (viele-r soziale-r). "of" isn't translated (the genitive fulfils the function of "of")
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(3) Beteiligst du dich an vielen sozialen Netzwerken? ~ roughly translating as "Do you take part in many social networks?"
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Is it okay if I correct your first sentence? - Instead of "Ich bin versucht zu sagen..." you should say "Ich versuche zu sagen..."